Point-of-sale (POS) based laser scanning system providing six-sided 360 degree omni-directional bar code symbol scanning coverage at a POS station

ABSTRACT

The laser scanning system includes a laser scanning plane generation mechanism disposed within a housing mounted in or on a countertop at the POS station. The mechanism generates first and second pluralities of laser scanning planes which (i) intersect within predetermined scan regions contained within a 3-D scanning volume defined outside of the housing, and (ii) generate a plurality of groups of intersecting laser scanning planes within the 3-D scanning volume. The plurality of groups of intersecting laser scanning planes form a complex omni\-directional 3-D laser scanning pattern within the 3-D scanning volume capable of scanning a bar code symbol located on the surface of any object including a six-sided rectangular box-shaped object, presented within the 3-D scanning volume at any orientation and from any direction at the POS station so as to provide six-sided 360-degree omni-directional bar code symbol scanning coverage at the POS station.

CROSS-REFERENCE TO RELATED U.S. APPLICATIONS

This is a Continuation of U.S. application Ser. No. 11/416,632 filed May3, 2006 now U.S. Pat. No. 7,422,156, which is a Continuation of U.S.application Ser. No. 10/911,397 filed Aug. 4, 2004, now U.S. Pat. No.7,100,832; which is a Continuation of U.S. application Ser. No.10/045,577 filed Jan. 11, 2002, now U.S. Pat. No. 6,918,540; which is aContinuation-in-Part of U.S. application Ser. No. 10/045,605 filed Jan.11, 2002, now U.S. Pat. No. 6,830,190; each said application being ownedby Assignee, Metrologic Instruments, Inc., of Blackwood, N.J., andincorporated herein by reference as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates generally to laser scanners ofultra-compact design capable of reading bar code symbols inpoint-of-sale (POS) and other demanding scanning environments.

2. Brief Description of the Prior Art

The use of bar code symbols for product and article identification iswell known in the art. Presently, various types of bar code symbolscanners have been developed. In general, these bar code symbol readerscan be classified into two distinct classes.

The first class of bar code symbol reader simultaneously illuminates allof the bars and spaces of a bar code symbol with light of a specificwavelength(s) in order to capture an image thereof for recognition anddecoding purposes. Such scanners are commonly known as CCD scannersbecause they use CCD image detectors to detect images of the bar codesymbols being read.

The second class of bar code symbol reader uses a focused light beam,typically a focused laser beam, to sequentially scan the bars and spacesof a bar code symbol to be read. This type of bar code symbol scanner iscommonly called a “flying spot” scanner as the focused laser beamappears as “a spot of light that flies” across the bar code symbol beingread. In general, laser bar code symbol scanners are sub-classifiedfurther by the type of mechanism used to focus and scan the laser beamacross bar code symbols.

The majority of laser scanners in use today, particular in retailenvironments, employ lenses and moving (i.e. rotating or oscillating)mirrors and/or other optical elements in order to focus and scan laserbeams across bar code symbols during code symbol reading operations. Indemanding retail scanning environments, it is common for such systems tohave both bottom and side-scanning windows to enable highly aggressivescanner performance, whereby the cashier need only drag a bar codedproduct past these scanning windows for the bar code thereon to beautomatically read with minimal assistance of the cashier or checkoutpersonal. Such dual scanning window systems are typically referred to as“bioptical” laser scanning systems as such systems employ two sets ofoptics disposed behind the bottom and side-scanning windows thereof.Examples of polygon-based bioptical laser scanning systems are disclosedin U.S. Pat. No. 4,229,588 and U.S. Pat. No. 4,652,732, assigned to NCR,Inc., each incorporated herein by reference in its entirety.

In general, prior art bioptical laser scanning systems are generallymore aggressive that conventional single scanning window systems. Forthis reason, bioptical scanning systems are often deployed in demandingretail environments, such as supermarkets and high-volume departmentstores, where high check-out throughput is critical to achieving storeprofitability and customer satisfaction.

While prior art bioptical scanning systems represent a technologicaladvance over most single scanning window system, prior art biopticalscanning systems in general suffered from various shortcomings anddrawbacks.

In particular, the laser scanning patterns of such prior art biopticallaser scanning systems are not optimized in terms of scanning coverageand performance, and are generally expensive to manufacture by virtue ofthe large number of optical components presently required to constructedsuch laser scanning systems.

Thus, there is a great need in the art for an improved bioptical-typelaser scanning bar code symbol reading system, while avoiding theshortcomings and drawbacks of prior art laser scanning systems andmethodologies.

Moreover, the performance of such aggressive laser scanning systems (inscanning a bar code symbol and accurately produce digital scan datasignals representative of a scanned bar code symbol) is susceptible tonoise, including ambient noise, thermal noise and paper noise. Morespecifically, during operation of such machines, a focused light beam isproduced from a light source such as a visible laser diode (VLD), andrepeatedly scanned across the elements of the code symbol attached,printed or otherwise fixed to the object to be identified. In the caseof bar code scanning applications, the elements of the code symbolconsists of a series of bar and space elements of varying width. Fordiscrimination purposes, the bars and spaces have different lightreflectivity (e.g. the spaces are highly light-reflective while the barsare highly light-absorptive). As the laser beam is scanned across thebar code elements, the bar elements absorb a substantial portion of thelaser beam power, whereas the space elements reflective a substantialportion thereof. As a result of this scanning process, the intensity ofthe laser beam is modulated to in accordance with the informationstructure encoded within the scanned bar code symbol. As the laser beamis scanned across the bar code symbol, a portion of the reflected lightbeam is collected by optics within the scanner. The collected lightsignal is subsequently focused upon a photodetector within the scannerwhich generates an analog electrical output signal which can bedecomposed into a number of signal components, namely a digital scandata signal having first and second signal levels, corresponding to thebars and spaces within the scanned code symbol; ambient-light noiseproduced as a result of ambient light collected by the light collectionoptics of the system; thermal noise produced as a result of thermalactivity within the signal detecting and processing circuitry; and“paper” or substrate noise produced as a result of the microstructure ofthe substrate in relation to the cross-sectional dimensions of thefocused laser scanning beam. The analog scan data signal haspositive-going transitions and negative-going transitions which signifytransitions between bars and spaces in the scanned bar code symbol.However, as a result of such noise components, the transitions from thefirst signal level to the second signal level and vice versa are notperfectly sharp, or instantaneous. Consequently, it is difficult todetermine the exact instant that each binary signal level transitionoccurs in detected analog scan data signal.

It is well known that the ability of a scanner to accurately scan a barcode symbol and accurately produce digital scan data signalsrepresentative of a scanned bar code symbol in noisy environmentsdepends on the depth of modulation of the laser scanning beam. The depthof modulation of the laser scanning beam, in turn, depends on severalimportant factors, namely: the ratio of the laser beam cross-sectionaldimensions at the scanning plane to the width of the minimal bar codeelement in the bar code symbol being scanned, and (ii) the signal tonoise ratio (SNR) in the scan data signal processing stage where binarylevel (1-bit) analog to digital (A/D) signal conversion occurs.

As a practical matter, it is not possible in most instances to produceanalog scan data signals with precisely-defined signal leveltransitions. Therefore, the analog scan data signal must be furtherprocessed to precisely determine the point at which the signal leveltransitions occur.

Hitherto, various circuits have been developed for carrying out suchscan data signal processing operations. Typically, signal processingcircuits capable of performing such operations include filters forremoving unwanted noise components, and signal thresholding devices forrejecting signal components which do not exceed a predetermined signallevel.

One very popular approach for converting analog scan data signals intodigital scan data signals is disclosed in U.S. Pat. No. 4,000,397,incorporated herein by reference in its entirety. In this US LettersPatent, a method and apparatus are disclosed for precisely detecting thetime of transitions between the binary levels of encoded analog scandata signals produced from various types of scanning devices. Accordingto this prior art method, the first signal processing step involvesdouble-differentiating the analog scan data input signal S_(analog) toproduce a second derivative signal S″_(analog). Then the zero-crossingsof the second derivative signal are detected, during selected gatingperiods, to signify the precise time at which each transition betweenbinary signal levels occurs. As taught in this US patent, the selectedgating periods are determined using a first derivative signalS′_(analog) formed by differentiating the input scan data signalS_(analog). Whenever the first derivative signal S′_(analog) exceeds athreshold level using peak-detection, the gating period is present andthe second derivative signal S″_(analog) is detected for zero-crossings.At each time instant when a second-derivative zero-crossing is detected,a binary signal level is produced at the output of the signal processor.The binary output signal level is a logical “1” when the detected signallevel falls below the threshold at the gating interval, and a logical“0” when the detected signal level falls above the threshold at thegating interval. The output digital signal S_(digital) produced by thissignal processing technique corresponds to the digital scan data signalcomponent contributing to the underlying structure of the analog scandata input signal S_(analog).

While the above-described signal processing technique generates a simpleway of generating a digital scan data signal from a corresponding analogscan data signal, this method has a number of shortcomings anddrawbacks.

In particular, thermal as well as “paper” or substrate noise imparted tothe analog scan data input signal S_(analog) tends to generatezero-crossings in the second-derivative signal S″_(analog) in much thesame manner as does binary signal level transitions encoded in the inputanalog scan data signal S_(analog). Consequently, the gating signalmechanism disclosed in U.S. Pat. No. 4,000,397 allows “false”second-derivative zero-crossing signals to be passed onto thesecond-derivative zero-crossing detector thereof, thereby producingerroneous binary signal levels at the output stage of this prior artsignal processor. In turn, error-ridden digital data scan data signalsare transmitted to the digital scan data signal processor of the barcode scanner for conversion into digital words representative of thelength of the binary signal levels in the digital scan data signal. Thiscan result in significant errors during bar code symbol decodingoperations, causing objects to be incorrectly identified and/orerroneous data to be entered into a host system.

Also, when scanning bar code symbols within a large scanning field withmultiple scanning planes that cover varying focal zones of the scanningfield, as taught in co-applicant's PCT International Patent PublicationNo. WO 97/22945 published on Jun. 26, 1997, Applicants' have observedthat the effects of paper/substrate noise are greatly amplified whenscanning bar code symbols in the near focal zone(s), thereby causing asignificant decrease in overall system performance. In the far out focalzones of the scanning system, Applicants have observed that laser beamspot speed is greatest and the analog scan data signals producedtherefrom are time-compressed relative to analog scan data signalsproduced from bar code symbols scanned in focal zones closer to thescanning system. Thus, in such prior art laser scanning systems,Applicants' have provided, between the first and second differentiatorstages of the scan data signal processor thereof, a low-pass filter(LHF) having cutoff frequency which passes (to the second differentiatorstage) the spectral components of analog scan data signals produced whenscanning bar code elements at the focal zone furthest out from thescanning system. While this technique has allowed prior art scanningsystems to scan bar codes in the far focal zones of the system, it hasin no way addressed or provided a solution to the problem of increasedpaper/substrate noise encountered when scanning bar code symbols in thenear focal zones of such laser scanning systems.

Moreover, although filters and signal thresholding devices are usefulfor rejecting noise components in the analog scan signal, such devicesalso limit the scan resolution of the system, potentially rendering thesystem incapable of reading low contrast and high resolution bar codesymbols on surfaces placed in the scanning field.

Thus, there is a great need in the art for improved laser scanningsystem wherein the analog scan data signals generated therewithin areprocessed so that the effects of thermal and paper noise encounteredwithin the system are significantly mitigated while not compromising thescan resolution of the system.

OBJECTS AND SUMMARY OF THE PRESENT INVENTION

Accordingly, a primary object of the present invention is to provide anovel bioptical laser scanning system which is free of the shortcomingsand drawbacks of prior art bioptical laser scanning systems andmethodologies.

Another object of the present invention is to provide a bioptical laserscanning system, wherein a plurality of pairs of quasi-orthogonal laserscanning planes are projected within predetermined regions of spacecontained within a 3-D scanning volume defined between the bottom andside-scanning windows of the system.

Another object of the present invention is to provide such a biopticallaser scanning system, wherein the plurality of pairs ofquasi-orthogonal laser scanning planes are produced using at least onerotating polygonal mirror having scanning facets that have high and lowelevation angle characteristics.

Another object of the present invention is to provide such a biopticallaser scanning system, wherein the plurality of pairs ofquasi-orthogonal laser scanning planes are produced using at least tworotating polygonal mirrors, wherein a first rotating polygonal mirrorproduces laser scanning planes that project from the bottom-scanningwindow, and wherein a second rotating polygonal mirror produces laserscanning planes that project from the side-scanning window.

Another object of the present invention is to provide such a biopticallaser scanning system, wherein each pair of quasi-orthogonal laserscanning planes comprises a plurality of substantially-vertical laserscanning planes for reading bar code symbols having bar code elements(i.e., ladder type bar code symbols) that are oriented substantiallyhorizontal with respect to the bottom-scanning window, and a pluralityof substantially-horizontal laser scanning planes for reading bar codesymbols having bar code elements (i.e., picket-fence type bar codesymbols) that are oriented substantially vertical with respect to thebottom-scanning window.

Another object of the present invention is to provide a bioptical laserscanning system comprising a plurality of laser scanning stations, eachof which produces a plurality of groups of quasi-orthogonal laserscanning planes that are projected within predetermined regions of spacecontained within a 3-D scanning volume defined between the bottom andside-scanning windows of the system.

Another object of the present invention is to provide a bioptical laserscanning system, wherein two visible laser diodes (VLDs) disposed onopposite sides of a rotating polygonal mirror are used to create aplurality of groups of quasi-orthogonal laser scanning planes thatproject through the bottom-scanning window.

Another object of the present invention is to provide a bioptical laserscanning system, wherein a single VLD is used to create the scan patternprojected through the side-scanning window.

Another object of the present invention is to provide a bioptical laserscanning system which generates a plurality of quasi-orthogonal laserscanning planes that project through the bottom-scanning window andside-scanning window to provide 360 degrees of scan coverage at a POSstation.

Another object of the present invention is to provide a bioptical laserscanning system which generates a plurality of vertical laser scanningplanes that project through the bottom-scanning window to provide 360degrees of scan coverage.

Another object of the present invention is to provide a bioptical laserscanning system which generates a plurality of horizontal and verticallaser scanning planes that project from the top of the side-scanningwindow downward, which are useful for reading ladder type andpicket-fence type bar code symbols on top-facing surfaces.

A further object of the present invention is to provide such a biopticallaser scanning system, in which an independent signal processing channelis provided for each laser diode and light collection/detectionsubsystem in order to improve the signal processing speed of the system.

A further object of the present invention is to provide such a biopticallaser scanning system, in which a plurality of signal processors areused for simultaneously processing the scan data signals produced fromeach of the photodetectors within the laser scanner.

A further object of the present invention is to provide a biopticallaser scanning system that provides improved scan coverage over thevolume disposed between the two scanning windows of the system.

Another object of the present invention is to provide a bioptical laserscanning system that produces horizontal scanning planes capable ofreading picket-fence type bar code symbols on back-facing surfaces whosenormals are substantially offset from the normal of the side-scanningwindow.

Another object of the present invention is to provide a bioptical laserscanning system that produces horizontal scanning planes that projectfrom exterior portions (for example, left side and right side) of theside-scanning window at a characteristic propagation direction whosenon-vertical component is greater than thirty-five degrees from normalof the side-scanning window.

Another object of the present invention is to provide a bioptical laserscanning system that produces vertical scanning planes capable ofreading ladder type bar code symbols on back-facing surfaces whosenormals are substantially offset from the normal of the side-scanningwindow.

Another object of the present invention is to provide a bioptical laserscanning system that produces a plurality a vertical scanning planesthat project from portions (e.g., back-left and back-right corners) ofthe bottom-scanning window proximate to the back of the bottom-scanningwindow and the bottom side of the side-scanning window.

Another object of the present invention is to provide a bioptical laserscanning system that produces vertical scanning planes capable of 360degree reading of ladder type bar code symbols (e.g., on bottom-,front-, left-, back- and/or right-facing surfaces of an article).

Another object of the present invention is to provide a bioptical laserscanning system that produces a plurality a vertical scanning planesthat project from each one of the four corners of the bottom-scanningwindow.

Another object of the present invention is to provide a bioptical laserscanning system that produces at least eight different vertical scanningplanes that project from the side-scanning window.

Another object of the present invention is to provide a bioptical laserscanning system that produces at least 13 different horizontal scanningplanes that project from the side-scanning window.

Another object of the present invention is to provide a bioptical laserscanning system that produces at least 20 different horizontal scanningplanes that project from the side-scanning window

Another object of the present invention is to provide a bioptical laserscanning system that produces at least 21 different scanning planes thatproject from the side-scanning window.

Another object of the present invention is to provide a bioptical laserscanning system that produces at least 28 different scanning planes thatproject from the side-scanning window.

Another object of the present invention is to provide a bioptical laserscanning system that produces at least seven different vertical scanningplanes that project from the bottom-scanning window.

Another object of the present invention is to provide a bioptical laserscanning system that produces at least 21 different horizontal scanningplanes that project from the bottom-scanning window.

Another object of the present invention is to provide a bioptical laserscanning system that produces at least 25 different scanning planes thatproject from the bottom-scanning window.

Another object of the present invention is to provide a bioptical laserscanning system with at least one laser beam production module thatcooperates with a rotating polygonal mirror and a plurality of laserbeam folding mirrors to produce a plurality of scanning planes thatproject through the window, wherein the incidence angle of the laserbeam produced by the laser beam production module is offset with respectto the axis of rotation of the rotating polygonal mirror.

A further object of the present invention is to provide such a biopticallaser scanning system wherein the offset of the incidence angle of thelaser and the axis of rotation of the rotating polygonal mirror producesoverlapping scanning ray patterns that are incident on at least onecommon mirror to provide a dense scanning pattern projecting therefrom.

In another aspect of the present invention, it is a primary objective toprovide an improved laser scanning system, wherein scan data signalsproduced therewithin are processed so that the effects of thermal andpaper noise encountered within the system are significantly mitigated.

Another object of the present invention is to provide an improved laserscanning system having a scan data signal processor with improveddynamic range.

Another object of the present invention is to provide an improved laserscanning system having a multi-path scan data signal processor thatemploys different operational characteristics (such as different filtercutoff frequencies, peak thresholds, etc) in distinct signal processingpaths.

Another object of the present invention is to provide an improved laserscanning system having a multi-path scan data signal processor thatconcurrently performs distinct signal processing operations that employdifferent operational characteristics (such as different filter cutofffrequencies, peak thresholds, etc).

Another object of the present invention is to provide an improved laserscanning system employing a scan data signal processor having aplurality of processing paths each processing the same data signalderived from the output of a photodetector to detect bar code symbolstherein and generate data representing said bar code symbols, whereinthe plurality of processing paths have different operationalcharacteristics (such as different filter cutoff frequencies, peakthresholds, etc).

A further object of the present invention is to provide such an improvedlaser scanning system wherein each signal processing path includes apeak detector that identifies time periods during which a firstderivative signal exceeds at least one threshold level, and wherein theat least one threshold level for one of the respective paths isdifferent than the at least one threshold level for another of therespective paths.

A further object of the present invention is to provide such an improvedlaser scanning system wherein each signal processing path performs lowpass filtering, wherein the cut-off frequency of such low pass filteringfor one of the respective paths is different than the cut-off frequencyof such low pass filtering for another of the respective paths.

A further object of the present invention is to provide such an improvedlaser scanning system wherein each signal processing path performsvoltage amplification, wherein the gain of such voltage amplificationfor one of the respective paths is different than the gain of suchvoltage amplification for another of the respective paths.

Another object of the present invention is to provide an improved laserscanning system employing a scan data signal processor with dynamic peakthreshold levels.

Another object of the present invention is to provide an improved laserscanning system employing a scan data signal processor with multiplesignal processing paths that perform analog signal processing functionswith analog circuitry.

Another object of the present invention is to provide an improved laserscanning system employing a scan data signal processor with multiplesignal processing paths that perform digital signal processing functionswith digital signal processing circuitry.

A further object of the present invention is to provide such a laserscanning system, wherein each processing path is performed sequentiallybased on real-time status of a working buffer that stores data valuesfor digital signal processing.

These and other objects of the present invention will become apparenthereinafter and in the Claims to Invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more fully understand the Objects of the Present Invention,the following Detailed Description of the Illustrative Embodimentsshould be read in conjunction with the accompanying Figure Drawings inwhich:

FIG. 1A is a perspective view of an illustrative embodiment of thebioptical laser scanning system of the present invention, showing itsbottom-scanning and side-scanning windows formed with its compactscanner housing.

FIG. 1B is a side view of the bioptical laser scanning system of FIG.1A.

FIG. 1C is a side view of the bioptical laser scanning system of FIGS.1A and 1B with an integrated weigh scale for use in a Point-Of-Sale(POS) retail environment as shown in FIG. 1H.

FIG. 1D is a front view of the bioptical laser scanning system withintegrated weigh scale of FIG. 1C.

FIG. 1E is a top view of the bioptical laser scanning system withintegrated weigh scale of FIG. 1C.

FIGS. 1F and 1G are perspective views of a model of the bioptical laserscanning system with integrated weigh scale of FIG. 1C.

FIG. 1H is a perspective view of the bioptical laser scanning system ofthe present invention shown installed in a Point-Of-Sale (POS) retailenvironment.

FIG. 1I is a perspective view of the bioptical laser scanning system ofthe present invention shown installed above a work surface (e.g. aconveyor belt structure) employed, for example, in manual sortationoperations or the like.

FIG. 1J is a pictorial illustration depicting a normal of a surface andthe “flip-normal” of the surface as used herein.

FIG. 1K is a pictorial illustration depicting bottom-facing, top-facing,back-facing, front-facing, left-facing and right-facing surfaces of arectangular shaped article oriented within the scanning volume of thebioptical laser scanning system of the present invention disposedbetween the bottom-scanning and side-scanning windows of the system.

FIG. 2A is a perspective view of a wire frame model of portions of thehorizontal section of the bioptical laser scanning system of theillustrative embodiment of the present invention, including thebottom-scanning window (e.g., horizontal window), first rotatingpolygonal mirror PM1, and the first and second scanning stations HST1and HST2 disposed thereabout, wherein each laser scanning stationincludes a set of laser beam folding mirrors disposed about the firstrotating polygon PM1.

FIG. 2B is a top view of the wire frame model of FIG. 2A.

FIG. 2C1 is a perspective view of a wire frame model of portions of thehorizontal section of the bioptical laser scanning system of theillustrative embodiment of the present invention, including thebottom-scanning window (e.g., horizontal window), first rotatingpolygonal mirror PM1, and the first and second scanning stations HST1and HST2 disposed thereabout, wherein each laser scanning stationincludes a light collecting/focusing optical element (labeled LC_(HST1)and LC_(HST2)) that collects light from a scan region that encompassesthe outgoing scanning planes and focuses such collected light onto aphotodetector (labeled PD_(HST1) and PD_(HST2)), which produces anelectrical signal whose amplitude is proportional to the intensity oflight focused thereon. The electrical signal produced by thephotodetector is supplied to analog/digital signal processing circuitry,associated with the first and second laser scanning station HST1 andHST2, that process analog and digital scan data signals derivedtherefrom to perform bar code symbol reading operations. Preferably, thefirst and second laser scanning stations HST1 and HST2 each include alaser beam production module (not shown) that generates a laser scanningbeam (labeled SB1 and SB2) that is directed through an aperture in thecorresponding light collecting/focusing element as shown to a point ofincidence on the first rotating polygonal mirror PM1.

FIG. 2C2 is a top view of the wire frame model of FIG. 2C1.

FIG. 2D is a perspective view of a wire frame model of portions of thevertical section of the bioptical laser scanning system of theillustrative embodiment of the present invention, including theside-scanning window (e.g., vertical window), second rotating polygonalmirror PM2, and the third scanning station VST1 disposed thereabout; thethird laser scanning station includes a set of laser beam foldingmirrors disposed about the second rotating polygon PM2.

FIG. 2E is a top view of the wire frame model of FIG. 2D.

FIG. 2F is a perspective view of a wire frame model of portions of thevertical section of the bioptical laser scanning system of theillustrative embodiment of the present invention, including theside-scanning window (e.g., vertical window), second rotating polygonalmirror PM2, and the third scanning stations VST1 disposed thereabout,wherein the third laser scanning station VST1 includes a lightcollecting/focusing optical element (labeled LC_(VST1)) that collectslight from a scan region that encompasses the outgoing scanning planesand focuses such collected light onto a photodetector (labeledPD_(VST1)), which produces an electrical signal whose amplitude isproportional to the intensity of light focused thereon. The electricalsignal produced by the photodetector is supplied to analog/digitalsignal processing circuitry, associated with the first and second laserscanning station HST1 and HST2, that process analog and digital scandata signals derived therefrom to perform bar code symbol readingoperations. Preferably, the third laser scanning station VST1 includes alaser beam production module (not shown) that generates a laser scanningbeam SB3 that is directed to a small light directing mirror disposed inthe interior of the light collecting/focusing element LC_(VST1) asshown, which redirects the laser scanning beam SB3 to a point ofincidence on the second rotating polygonal mirror PM2.

FIG. 2G1 depicts the angle of each facet of the rotating polygonalmirrors PM1 and PM2 with respect to the rotational axis of therespective rotating polygonal mirrors in the illustrative embodiment ofthe present invention.

FIG. 2G2 is a pictorial illustration of the scanning ray patternproduced by the four facets of the first polygonal mirror PM1 inconjunction with the laser beam source provided by the first laserscanning station HST1 in the illustrative embodiment of the presentinvention is shown in FIG. 2G2. A similar scanning ray pattern isproduced by the four facets of the first polygonal mirror PM1 inconjunction with the laser beam source provided by the second laserscanning station HST2.

FIG. 2G3 is a pictorial illustration of the scanning ray patternproduced by the four facets of the second polygonal mirror PM2 inconjunction with the laser beam source provided by the third laserscanning station VST1 in the illustrative embodiment of the presentinvention; the facets of the second polygonal mirror PM2 can bepartitioned into two classes: a first class of facets (corresponding toangles β₁ and β₂) have High Elevation (HE) angle characteristics, and asecond class of facets (corresponding to angles β₃ and β₄) have LowElevation (LE) angle characteristics; high and low elevation anglecharacteristics are referenced by the plane P1 that contains theincoming laser beam and is normal to the rotational axis of the secondpolygonal mirror PM2; each facet in the first class of facets (havinghigh beam elevation angle characteristics) produces an outgoing laserbeam that is directed above the plane P1 as the facet sweeps across thepoint of incidence of the third laser scanning station VST1; whereaseach facet in the second class of facets (having low beam elevationangle characteristics) produces an outgoing laser beam that is directedbelow the plane P1 as the facet sweeps across the point of incidence ofthe third laser scanning station VST1.

FIG. 2H depicts the offset between the pre-specified angle of incidenceof the laser beams produced by the laser beam production modules of thelaser scanning stations HST1 and HST2 and the rotational axis of thepolygonal mirror PM1 along a direction perpendicular to the rotationalaxis; Such offset provides for spatial overlap in the scanning patternof light beams produced from the polygonal mirror PM1 by these laserbeam production modules; such spatial overlap can be exploited such thatthe overlapping rays are incident on at least one common mirror (mh5 inthe illustrative embodiment) to provide a dense scanning patternprojecting therefrom; in the illustrative embodiment, a dense pattern ofhorizontal planes (groups GH4) is projected from the front side of thebottom window as is graphically depicted in FIGS. 3F1, 3F2 and 4B1 and4B2.

FIG. 3A illustrates the intersection of the four groups of laserscanning planes (with 20 total scanning planes in the four groups)produced by the first laser scanning station HST1 on the bottom-scanningwindow 16 in the illustrative embodiment of the present invention.

FIGS. 3B1 and 3B2 graphically depict a vector-based nomenclature thatmay be used to define horizontal and vertical scanning planes,respectively, that project through the bottom-scanning window 16.

FIGS. 3C1 and 3C2 is a perspective view and top view, respectively, of awire frame model that illustrates the first group GH1 of laser beamfolding mirrors of the first laser scanning station (HST1), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the right back corner of the bottom-scanningwindow 16 diagonally outward and upward above the front left side (andfront left corner) of the bottom-scanning window 16 as shown.

FIGS. 3D1 and 3D2 is a front view and top view, respectively, of a wireframe model that illustrates the second group GH2 of laser beam foldingmirrors of the first laser scanning station (HST1), which cooperate withthe four scanning facets of the first rotating polygonal mirror PM1 soas to generate four different horizontal laser scanning planes thatproject from the right side of the bottom-scanning window 16 diagonallyoutward and upward above the left side of the bottom-scanning window 16as shown.

FIGS. 3E1 and 3E2 is a perspective view and top view, respectively of awire frame model that illustrates the third group GH3 of laser beamfolding mirrors of the first laser scanning station (HST1), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the right front corner of the bottom-scanningwindow 16 diagonally outward and upward above the back left side andback left corner of the bottom-scanning window 16 as shown.

FIGS. 3F1 and 3F2 is a front view and side view, respectively, of a wireframe model that illustrates the fourth group GH4 of laser beam foldingmirrors of the first laser scanning station (HST1), which cooperate withthe four scanning facets of the first rotating polygonal mirror PM1 soas to generate eight different horizontal laser scanning planes thatproject from the front side of the bottom-scanning window 16 diagonallyoutward and upward above the back side of the bottom-scanning window 16as shown; note that the first laser scanning station HST1 utilizesmirrors MH4 and MH5 (and not MH6) of group GH4 to produce eightdifferent scan planes therefrom.

FIG. 4A illustrates the intersection of the four groups of laserscanning planes (with 20 total scanning planes in the four groups)produced by the second laser scanning station HST2 on thebottom-scanning window 16 in the illustrative embodiment of the presentinvention.

FIGS. 4B1 and 4B2 is a front view and side view, respectively, of a wireframe model that illustrates the first group (GH4) of laser beam foldingmirrors of the second laser scanning station (HST2), which cooperatewith the four scanning facets of the first rotating polygonal mirror PM1so as to generate eight different horizontal laser scanning planes thatproject from the front side of the bottom-scanning window 16 diagonallyoutward and upward above the back side of the bottom-scanning window 16as shown; note that the second laser scanning station HST2 utilizesmirrors MH5 and MH6 (and not MH4) of group GH4 to produce eightdifferent scan planes therefrom.

FIGS. 4C1 and 4C2 is a perspective view and top view, respectively, of awire frame model that illustrates the second group (GH5) of laser beamfolding mirrors of the second laser scanning station (HST2), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the left front corner of the bottom-scanningwindow 16 diagonally outward and upward above the back right side andback right corner of the bottom-scanning window 16 as shown.

FIGS. 4D1 and 4D2 is a front view and top view, respectively, of a wireframe model that illustrates the third group (GH6) of laser beam foldingmirrors of the second laser scanning station (HST2), which cooperatewith the four scanning facets of the first rotating polygonal mirror PM1so as to generate four different horizontal laser scanning planes thatproject from the left side of the bottom-scanning window 16 diagonallyoutward and upward above the right side of the bottom-scanning window 16as shown.

FIGS. 4E1 and 4E2 is a perspective view and top-view, respectively, of awire frame model that illustrates the fourth group (GH7) of laser beamfolding mirrors of the second laser scanning station (HST2), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the left back corner of the bottom-scanningwindow 16 diagonally outward and upward above the front right side andfront right corner of the bottom-scanning window 16 as shown.

FIG. 4F illustrates the vertical scanning planes that project from thebottom-scanning window 16; including 4 groups (namely, GH1, GH3, GH5 andGH7); groups GH1 and GH5 project from opposing portions (e.g., theback-right and front-left corners of the window 16) of thebottom-scanning window 16, and groups GH3 and GH7 project from opposingportions (e.g., front-right and back-left corners of the window 16) ofthe bottom-scanning window; note that groups GH1 and GH5 aresubstantially co-planar (i.e., quasi co-planar) and groups GH3 and GH7are substantially co-planar (i.e., quasi co-planar), while groups GH1and GH5 are substantially orthogonal (i.e., quasi-orthogonal) to groupsGH3 and GH7, respectively, as shown.

FIG. 5A illustrates the intersection of the fourteen groups of laserscanning planes (with 28 total scanning planes in the fourteen groups)produced by the third laser scanning station VST1 on the side-scanningwindow 18 in the illustrative embodiment of the present invention.

FIGS. 5B1 and 5B2 graphically depict a vector-based nomenclature thatmay be used to define horizontal and vertical scanning planes,respectively, that project through the side-scanning window 18.

FIGS. 5C1 and 5C2 is a front view and top view, respectively, of a wireframe model that illustrates the first group (GV1) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two low-elevation (LE) scanning facets of the second rotatingpolygonal mirror PM2 (corresponding to angles β₃ and β₄ of the secondpolygonal mirror PM2 in FIG. 2G1) so as to generate two differentvertical laser scanning planes that project from the left side of theside-scanning window 18 diagonally down and out across thebottom-scanning window 16 above the front right corner of thebottom-scanning window 16 as shown.

FIGS. 5D1 and 5D2 is a perspective view and side view, respectively, ofa wire frame model that illustrates the second group (GV2) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low-elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differentvertical laser scanning planes that project from the top left corner ofthe side-scanning window 18 downward toward the bottom-scanning window16 substantially along the left side of the bottom-scanning window 16 asshown.

FIGS. 5E1 and 5E2 is a front view and side view, respectively, of a wireframe model that illustrates the third group (GV3) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two low-elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₃ and β₄ of the second polygonalmirror PM2 in FIG. 2G1) so as to generate two different horizontal laserscanning planes that project from the top left quadrant of theside-scanning window 18 diagonally down across the bottom-scanningwindow 16 as shown.

FIGS. 5F1 and 5F2 is a front view and side view, respectively, of a wireframe model that illustrates the fourth group (GV4) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the top rightquadrant of the side-scanning window 18 diagonally down across thebottom-scanning window 16 as shown.

FIGS. 5G1 and 5G2 is a front view and side view, respectively, of a wireframe model that illustrates the fifth group (GV5) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two low-elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₃ and β₄ of the second polygonalmirror PM2 in FIG. 2G1) so as to generate two different vertical laserscanning planes that project from the top right corner of theside-scanning window 18 downward toward the bottom-scanning window 16substantially along the right side of the bottom-scanning window 16 asshown.

FIGS. 5H1 and 5H2 is a front view and side view, respectively, of a wireframe model that illustrates the sixth group (GV6) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two low elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₃ and β₄ of the second polygonalmirror PM2 in FIG. 2G1) so as to generate two different vertical laserscanning planes that project from the right side of the side-scanningwindow 18 diagonally out across the bottom-scanning window 16 above thefront left corner of the bottom-scanning window 16 as shown.

FIGS. 5I1 and 5I2 is a front view and side view, respectively, of a wireframe model that illustrates the seventh group (GV7) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the top left quadrantof the side-scanning window 18 outwardly across the bottom-scanningwindow 16 (substantially parallel to the bottom-scanning window 16) asshown.

FIGS. 5J1 and 5J2 is a front view and top view, respectively, of a wireframe model that illustrates the eighth group (GV8) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the left side of theside-scanning window 18 outwardly across the bottom-scanning window 16(substantially parallel to the bottom-scanning window 16) as shown; inthe illustrative embodiment, the characteristic direction of propagationof such scanning planes has a non-vertical component whose orientationrelative to the normal of the side-scanning window 18 is greater than 35degrees.

FIGS. 5K1 and 5K2 is a front view and side view, respectively, of a wireframe model that illustrates the ninth group (GV9) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two high elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₁ and β₂ of the second polygonalmirror PM2 in FIG. 2G1) so as to generate two different horizontal laserscanning planes that project from the central portion of theside-scanning window 18 outwardly and downward across thebottom-scanning window 16 as shown.

FIGS. 5L1 and 5L2 is a front view and side view, respectively, of a wireframe model, that illustrates the tenth group (GV10) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the central portionof the side-scanning window 18 outwardly and sharply downward across thebottom-scanning window 16 as shown.

FIGS. 5M1 and 5M2 is a front view and side view, respectively, of a wireframe model that illustrates the eleventh group (GV11) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the central portionof the side-scanning window 18 outwardly and sharply downward across thebottom-scanning window 16 as shown.

FIGS. 5N1 and 5N2 is a front view and side view, respectively, of a wireframe model that illustrates the twelfth group (GV12) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the central portionof the side-scanning window 18 outwardly across the bottom-scanningwindow 16 (substantially parallel to the bottom-scanning window 16) asshown.

FIGS. 5O1 and 5O2 is a front view and top view, respectively, of a wireframe model that illustrates the thirteenth group (GV13) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the right side of theside-scanning window 18 outwardly across the bottom-scanning window 16(substantially parallel to the bottom-scanning window 16) as shown; inthe illustrative embodiment, the characteristic direction of propagationof such scanning planes has a non-vertical component whose orientationrelative to the normal of the side-scanning window 18 is greater than 35degrees.

FIGS. 5P1 and 5P2 is a front view and side view, respectively, of a wireframe model that illustrates the fourteenth group (GV14) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the top rightquadrant of the side-scanning window 18 outwardly across thebottom-scanning window 16 (substantially parallel to the bottom-scanningwindow 16) as shown.

FIG. 6 is an exemplary timing scheme for controlling the bioptical laserscanner of the illustrative embodiment to cyclically generate a complexomni-directional 3-D laser scanning pattern from both the bottom andside-scanning windows 16 and 18 thereof during the revolutions of thescanning polygonal mirrors PM1 and PM2; in this exemplary timing scheme,four sets of scan plane groups (4*[GH1 . . . GH7]) are produced bystations HST1 and HST2 during each revolution of the polygonal mirrorPM1 concurrently with two sets of scan plane groups (2*[GV1 . . . GV14])produced by station VST1 during a single revolution of the polygonalmirror PM2; this complex omni-directional scanning pattern isgraphically illustrated in FIGS. 3A through 5P2; the 3-D laser scanningpattern of the illustrative embodiment consists of 68 different laserscanning planes, which cooperate in order to generate a plurality ofquasi-orthogonal laser scanning patterns within the 3-D scanning volumeof the system, thereby enabling true omnidirectional scanning of barcode symbols.

FIG. 7 is a functional block diagram of an illustrative embodiment ofthe electrical subsystem of the bioptical laser scanning systemaccording to the present invention, including: photodetectors (e.g. asilicon photocell) for detection of optical scan data signals generatedby the respective laser scanning stations; analog signal processingcircuitry for processing (e.g., preamplification, bandpass filtering,and A/D conversion) analog scan data signals, digitizing circuitry forconverting the digital scan data signal D₂, associated with each scannedbar code symbol, into a corresponding sequence of digital words (i.e. asequence of digital count values) D₃, and bar code symbol decodingcircuitry that receives the digital word sequences D₃ produced from thedigitizing circuit, and subject it to one or more bar code symboldecoding algorithms in order to determine which bar code symbol isindicated (i.e. represented) by the digital word sequence D₃; aprogrammed microprocessor 61 with a system bus and associated programand data storage memory, for controlling the system operation of thebioptical laser scanner and performing other auxiliary functions and forreceiving bar code symbol character data (provided by the bar codesymbol decoding circuitry); a data transmission subsystem forinterfacing with and transmitting symbol character data and otherinformation to host computer system (e.g. central computer, cashregister, etc.) over a communication link therebetween; and aninput/output interface for providing drive signals to anaudio-transducer and/or LED-based visual indicators used to signalsuccessful symbol reading operations to users and the like, forproviding user input via interaction with a keypad, and for interfacingwith a plurality of accessory devices (such as an external handheldscanner, a display device, a weigh scale, a magnetic card reader and/ora coupon printer as shown).

FIGS. 8A and 8B are graphical representations of the power spectrum ofan exemplary analog scan data signal produced when laser scanning a barcode symbol within near and far focal zones of a laser scanning system,shown plotted along with the power density spectrum of thepaper/substrate noise signal produced while laser scanning the bar codesymbol on its substrate within such near and far focal zones.

FIG. 9 is a functional block diagram of an illustrative embodiment ofthe multi-path scan data signal processor according to the presentinvention, including: signal conditioning circuitry 903 operably coupledbetween a photodetector 902 and a plurality of signal processing paths(two shown as path A and path B) that process the output of the signalconditioning circuitry in parallel; each signal processing pathincludes: a first derivative signal generation circuit 904 having adifferentiator, low pass filter and amplifier therein; a secondderivative signal generation circuit 906 having a differentiatortherein; a first derivative signal threshold-level generation circuit905; and a zero crossing detector 907, data gate 908, and binary-typeA/D signal conversion circuitry 909; each signal processing path hasdifferent operational characteristics (such as different cutofffrequencies in the filtering stages of the first and second derivativesignal generation circuits of the respective paths, different gaincharacteristics in amplifier stages of the first and second derivativesignal generation circuits of the respective paths, and/or differentpositive and negative signal thresholds in the first derivativethreshold circuitry of the respective paths); the varying operationalcharacteristics of the paths provide different signal processingfunctions.

FIGS. 10A through 10I are signal diagrams that illustrate the operationof the multi-path scan data signal processor 901 of the illustrativeembodiment of FIG. 9. FIGS. 10A and 10B depict the signal produced atthe output of the photodetector 902 as the laser scanning beam scansacross a bar code symbol; FIG. 10C depicts the output signal produced bythe signal conditioning circuitry 903; and FIGS. 10D through 10I depictthe processing performed in one of the respective paths of themulti-path scan data signal processor 901; similar processing operationswith different operations characteristics are performed in other pathsof the multi-path scan data signal processor 901.

FIGS. 11A and 11B, taken together, set forth a schematic diagramillustrating an exemplary embodiment of the signal conditioningcircuitry 903 of FIG. 9, which operates to amplify and smooth out orotherwise filter the scan data signal produced by the photodetector 902to remove unwanted noise components therein, including a number ofsubcomponents arranged in a serial manner, namely: a high gain amplifierstage 1103, a multistage amplifier stage 1105, a differential amplifierstage 1107 and a low pass filter (LPF) stage 1109.

FIG. 12 is a schematic diagram illustrating an exemplary implementationof the first derivative signal generation circuitry 904, which issuitable for use in the two different paths of the scan data signalprocessor of FIG. 9, including a number of subcomponents arranged in aserial manner that process the analog scan data signal produced by thesignal conditioning circuitry 903, namely: a differentiator stage 1201,a low-pass filter (LPF) stage 1203, and an amplifier stage 1205.

FIG. 13 is a schematic diagram illustrating an exemplary implementationof the second derivative signal generation circuitry 906, which issuitable for use in the two different paths of the scan data signalprocessor of FIG. 9, including: a differentiator stage 1301 thatgenerates a signal whose voltage level is proportional to the derivativeof the first derivative signal produced by the first derivativegeneration circuitry 904 (thus proportional to the second derivative ofthe analog scan data signal produced by the signal conditioningcircuitry 903) for frequencies in a predetermined frequency band.

FIGS. 14A through 14C set forth a schematic diagram illustrating anexemplary implementation of the first derivative signal thresholdcircuitry 905, which is suitable for use in the two different paths ofthe scan data signal processor of FIG. 9, including: an amplifier stage1401 that amplifies the voltage levels of the first derivative signalproduced by the first derivative signal generation circuitry 904,positive and negative peak detectors 1403 and 1405, and a comparatorstage 1407 that generates output signals (e.g., the Upper_ThresholdSignal and Lower_Threshold Signal) that indicate the time period whenthe positive and negative peaks of the amplified first derivative signalproduced by the amplifier stage exceed predetermined thresholds (i.e., apositive peak level PPL and a negative peak level NPL).

FIG. 15 illustrates an exemplary implementation of a zero crossingdetector 907, which is suitable for use in the two different paths ofthe scan data signal processor of FIG. 9, including a comparator circuitthat compares the second derivative signal produced from the secondderivative generation circuit in its respective path with a zero voltagereference (i.e. the AC ground level) provided by the zero referencesignal generator, in order to detect the occurrence of eachzero-crossing in the second derivative signal, and provide outputsignals (ZC_1 and ZC_2 signals) identifying zero crossings in the secondderivative signal.

FIG. 16 is a schematic diagram illustrating an exemplary implementationof the data gating circuitry 908 and 1-Bit A/D conversion circuitry 909,which is suitable for use in the two different paths of the scan datasignal processor of FIG. 9.

FIG. 17A is a functional block diagram of a system architecture suitablefor a digital implementation of the scan data signal processor of thepresent invention, including: signal conditioning circuitry 1703 (whichamplifies and filters the analog signal to remove unwanted noisecomponents as described above), analog-to-digital conversion circuitry1705 which samples the conditioned analog scan data signals at asampling frequency at least two times the highest frequency componentexpected in the analog scan data signal, in accordance with the wellknown Nyquist criteria, and quantizes each time-sampled scan data signalvalue into a discrete signal level using a suitable length numberrepresentation (e.g. 8 bits) to produce a discrete scan data signal; andprogrammed processor (e.g., a digital signal processor 1707 andassociated memory 1709 as shown) that processes the discrete signallevels to generate a sequence of digital words (i.e. a sequence ofdigital count values) D₃, each representing the time length associatedwith the signal level transitions in the corresponding digital scan datasignal as described above.

FIGS. 17B through 17D are functional block diagrams that illustrateexemplary digital implementations of the multi-path scan data processingaccording to the present invention, wherein digital signal processingoperations are preferably carried out on the discrete scan data signallevels generated by the A/D converter 1705 and stored in the memory 1709of FIG. 17A; FIG. 17B illustrates exemplary digital signal processingoperations that identify a data frame (e.g., a portion of the discretescan data signal levels stored in memory 1709) that potentiallyrepresents a bar code symbol (block 1723) and stores the data frame in aworking buffer (block 1725); FIG. 17C illustrates exemplary digitalsignal processing operations that carry out multi-path scan data signalprocessing according to the present invention; and FIG. 17D illustratesalternative digital signal processing operations that carry outmulti-subpath scan data signal processing (with different firstderivative threshold processing performed in each subpath) according tothe present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS OF THE PRESENTINVENTION

Referring to the figures in the accompanying Drawings, the variousillustrative embodiments of the bioptical laser scanner of the presentinvention will be described in great detail.

In the illustrative embodiments, the apparatus of the present inventionis realized in the form of an automatic code symbol reading systemhaving a high-speed bioptical laser scanning mechanism as well as a scandata processor for decode processing scan data signals produced thereby.However, for the sake of convenience of expression, the term “biopticallaser scanner” shall be used hereinafter to denote the bar code symbolreading system which employs the bioptical laser scanning mechanism ofthe present invention.

As shown in FIGS. 1A through 1G, the bioptical laser scanner 1 of theillustrative embodiment of the present invention has a compact housing 2having a first housing portion 4A and a second housing portion 4B whichprojects from one end of the first housing portion 4A in a substantiallyorthogonal manner. When the laser scanner 1 is installed within acounter-top surface, as shown in FIG. 1H, the first housing portion 4Aoriented horizontally, whereas the second housing portion 4B is orientedvertically with respect to the POS station. Thus throughout theSpecification and Claims hereof, the terms first housing portion andhorizontally-disposed housing portion may be used interchangeably butrefer to the same structure; likewise, the terms the terms secondhousing portion and vertically-disposed housing portion may be usedinterchangeably but refer to the same structure.

In the illustrative embodiment, the first housing portion 4A (whichincludes the bottom-scanning window 16) has width, length and heightdimensions of 11.405, 14.678 and 3.93 inches, respectively, whereas thesecond housing portion 4B (which includes the side-scanning window 18)has width and height dimensions of 12.558 inches and 7.115 inches,respectively. The total height of the scanner housing 2 is 11.044inches. In addition, the bottom-scanning window 16 has width and lengthdimensions of approximately 3.94 inches (100 mm) and 5.9 inches (150mm), respectively, to provide a window with a square area ofapproximately 15,000 square mm. And, the side-scanning window 18 haswidth and height dimensions of approximately 9.8 inches (248 mm) and 5.9inches (150 mm), respectively, to provide a window with a square area ofapproximately 37,200 square mm. As will be described in greater detailbelow, the bioptical laser scanning mechanism housed within thisultra-compact housing produces an omni-directional laser scanningpattern within the three-dimensional volume above the bottom-scanningwindow 16 and in front of the side-scanning window 18.

The omni-directional scanning pattern is capable of reading picket-fencetype bar code symbols on bottom-facing surfaces (i.e., a surface whosenormal is directed toward the bottom-scanning window 16 of the scanner),top-facing surfaces (i.e., a surface whose “flip-normal” is directedtoward the bottom-scanning window 16 of the scanner), back-facingsurfaces (i.e., a surface whose normal is directed toward theside-scanning window 18 of the scanner), front-facing surfaces (i.e., asurface whose “flip-normal” is directed toward the side-scanning window18 of the scanner), left-facing surfaces (i.e., a surface whose normalis directed toward or above the left side of the scanner), andright-facing surfaces (i.e., a surface whose normal is directed towardor above the right side of the scanner). A “flip-normal” as used aboveis a direction co-linear to the normal of a surface yet opposite indirection to this normal as shown in FIG. 1J. An example of suchbottom-facing, top-facing, back-facing, front-facing surfaces,left-facing surfaces, and right-facing surfaces of a rectangular shapedarticle oriented in the scan volume of the bioptical laser scanningsystem disposed between bottom-scanning and side-scanning windows of thesystem is illustrated in FIG. 1K.

The bioptical laser scanning system of the present invention can be usedin a diverse variety of bar code symbol scanning applications. Forexample, the bioptical laser scanner 1 can be installed within thecountertop of a point-of-sale (POS) station as shown in FIG. 2H. In thisapplication, it is advantageous to integrate a weight scale with thelaser scanning mechanism. Such a device is shown in FIGS. 1C and 1Dincluding a weight transducer 35 affixed to a flange 37 mounted to thefront side of the first housing portion 4A. A weigh platter 31 (uponwhich goods or articles to be weighed are placed) is mechanicallysupported via posts (preferably disposed on its periphery) to a plasticinsert 33. The posts (not shown) and plastic insert 33 transfer theweight forces exerted by the goods or articles placed on the weighplatter 31 to the weight transducer 35 for measurement. In addition, theplastic insert 33 supports the bottom-scanning window 16.

As shown in FIG. 1H, the bioptical laser scanner 1 can be installedwithin the countertop of a point-of-sale (POS) station 26, having acomputer-based cash register 20, a weigh-scale 22 mounted within thecounter adjacent the laser scanner, and an automated transactionterminal (ATM) supported upon a courtesy stand in a conventional manner.

Alternatively, as shown in FIG. 1I, the bioptical laser scanner can beinstalled above a conveyor belt structure as part of a manually-assistedparcel sorting operation being carried out, for example, duringinventory control and management operations.

As shown in FIGS. 2A through 2F, the bioptical scanning system 1 of theillustrative embodiment includes two sections: a first section(sometimes referred to as the horizontal section) disposed within thefirst housing portion 4A and a second section (sometimes referred to asthe vertical section) substantially disposed within the second housingportion 4B. It should be noted that in the illustrative embodiment,parts of the vertical section are disposed within the back of the firsthousing portion 4A as will become evident from the figures andaccompanying description that follows.

As shown in FIGS. 2A through 2C2 (and in tables I and II below), thefirst section includes a first rotating polygonal mirror, and first andsecond scanning stations (indicated by HST1 and HST2, respectively)disposed thereabout. The first and second laser scanning stations HST1and HST2 each include a laser beam production module (not shown), a setof laser beam folding mirrors, a light collecting/focusing mirror; and aphotodetector. The first and second laser scanning stations HST1 andHST2 are disposed opposite one another about the first rotatingpolygonal mirror PM1. Each laser scanning station generates a laserscanning beam (shown as SB1 and SB2 in FIGS. 2C1 and 2C2) that isdirected to a different point of incidence on the first rotatingpolygonal mirror PM1. The incident laser beams (produced by the firstand second laser scanning stations HST1 and HST2) are reflected by eachfacet (of the first polygonal mirror PM1) at varying angles as the firstpolygonal mirror PM1 rotates to produce two scanning beams (SB1 and SB2)whose direction varies over the rotation cycle of the first polygonalmirror PM1. The first and second laser scanning stations HST1 and HST2include groups of laser beam folder mirrors arranged about the firstpolygonal mirror PM1 so as to redirect the two scanning beams SB1 andSB2 to thereby generate and project different groups of laser scanningplanes through the bottom-scanning window 16.

TABLE I Mirror Positions - Horizontal Section (mm): Vertex X Y Z mh1 1115.25 18.87 3.06 2 109.09 9.19 42.85 3 99.81 69.42 40.73 4 105.97 79.100.94 5 6 7 8 mh2 1 123.91 −78.90 2.61 2 95.43 −62.89 39.73 3 95.43 3.5739.73 4 123.91 19.57 2.61 5 6 7 8 mh3 1 103.74 −140.29 25.40 2 96.02−133.84 47.43 3 99.04 −68.09 37.13 4 114.48 −80.98 −6.92 5 112.97−113.85 −1.78 6 7 8 mh4 1 62.08 −136.87 −11.25 2 66.99 −152.92 31.34 326.71 −165.23 31.34 4 21.80 −149.19 −11.25 5 6 7 8 mh5 1 −20.00 −135.31−11.19 2 −20.00 −148.24 27.91 3 20.00 −148.24 27.91 4 20.00 −135.31−11.19 5 6 7 8 mh6 1 −62.08 −136.87 −11.25 2 −66.99 −152.92 31.34 3−26.71 −165.23 31.34 4 −21.80 −149.19 −11.25 5 6 7 8 mh7 1 −96.02−133.84 47.43 2 −99.04 −68.09 37.13 3 −114.48 −80.98 −6.92 4 −112.97−113.85 −1.78 5 −103.74 −140.29 25.40 6 7 8 mh8 1 −123.91 −78.90 2.61 2−95.43 −62.89 39.73 3 −95.43 3.57 39.73 4 −123.91 19.57 2.61 5 6 7 8 mh91 −115.25 18.87 3.06 2 −109.09 9.19 42.85 3 −99.81 69.42 40.73 4 −105.9779.10 0.94 5 6 7 8 mh10 1 53.69 23.10 −11.94 2 14.23 28.69 8.47 3 47.5467.87 24.47 4 72.59 81.43 24.47 5 102.20 77.24 9.16 6 106.06 65.68 −1.177 83.67 39.33 −11.94 8 mh11 1 123.91 −79.28 2.61 2 75.02 −71.42 −10.49 375.02 11.97 −10.49 4 123.91 19.83 2.61 5 6 7 8 mh12 1 116.06 −105.01−10.87 2 43.62 −99.13 −10.90 3 65.09 −142.38 30.61 4 101.96 −145.3730.63 5 6 7 8 mh13 1 −101.96 −145.37 30.63 2 −65.09 −142.38 30.61 3−43.62 −99.13 −10.90 4 −116.06 −105.01 −10.87 5 6 7 8 mh14 1 −75.0211.97 −10.49 2 −75.02 −71.42 −10.49 3 −123.91 −79.28 2.61 4 −123.9119.83 2.61 5 6 7 8 mh15 1 −54.15 22.24 −10.80 2 −84.14 38.47 −10.80 3−106.53 64.81 −0.04 4 −102.66 76.38 10.30 5 −73.05 80.57 25.61 6 −48.0067.01 25.61 7 −14.70 27.83 9.60 8

TABLE II Scan Line Groups - Horizontal Section Mirrors Scanning Station/Group Identifier in Group Scan Lines Type gh1 mh1, mh10 HST1/4 verticalgh2 mh2, mh11 HST1/4 horizontal gh3 mh3, mh12 HST1/4 vertical gh4 mh4HST1/4 horizontal mh5 HST1, HST2/8 mh6 HST2/4 gh5 mh7, mh13 HST2/4vertical gh6 mh8, HST2/4 horizontal mh14 gh7 mh9, mh15 HST2/4 vertical

In addition, as shown in FIGS. 2C1 and 2C2, the first and second laserscanning stations HST1 and HST2 each include a light collecting/focusingoptical element, e.g. parabolic light collecting mirror or parabolicsurface emulating volume reflection hologram (labeled LC_(HST1) andLC_(HST2)) that collects light from a scan region that encompasses theoutgoing scanning planes (produced by the first and second laserscanning stations HST1 and HST2) and focuses such collected light onto aphotodetector (labeled PD_(HST1) and PD_(HST2)), which produces anelectrical signal whose amplitude is proportional to the intensity oflight focused thereon. The electrical signal produced by thephotodetector is supplied to analog/digital signal processing circuitry,associated with the first and second laser scanning station HST1 andHST2, that process analog and digital scan data signals derivedtherefrom to perform bar code symbol reading operations. Preferably, thefirst and second laser scanning stations HST1 and HST2 each include alaser beam production module (not shown) that generates a laser scanningbeam (labeled SB1 and SB2) that is directed (preferably through anaperture in the corresponding light collecting/focusing element as shownin FIGS. 2C1 and 2C2) to a point of incidence on the first rotatingpolygonal mirror PM1.

As shown in FIGS. 2D through 2F and in tables III and IV below, thesecond section includes a second rotating polygonal mirror PM2 and athird scanning station (denoted VST1) that includes a laser beamproduction module (not shown), a set of laser beam folding mirrors, alight collecting/focusing mirror, and a photodetector. The third laserscanning station VST1 generates a laser scanning beam (labeled as SB3 inFIG. 2F) that is directed to a point of incidence on the second rotatingpolygonal mirror PM2. The incident laser beam is reflected by each facet(of the second polygonal mirror PM2) at varying angles as the secondpolygonal mirror PM2 rotates to produce a scanning beam whose directionvaries over the rotation cycle of the second polygonal mirror PM2. Thethird laser scanning station VST1 includes a set of laser beam foldermirrors arranged about the second rotating polygonal mirror PM2 so as toredirect the scanning beam to thereby generate and project differentgroups of laser scanning planes through the side-scanning window 18.

TABLE III Mirror Positions - Vertical Section (mm): Vertex X Y Z mv1 1−74.79 88.94 −10.38 2 −114.09 88.94 16.17 3 −114.09 154.82 16.17 4−74.79 154.82 −10.38 5 6 7 8 mv2 1 −61.12 131.03 −6.76 2 −77.92 146.4225.78 3 −43.75 183.72 25.78 4 −33.41 174.24 5.74 5 −31.44 163.43 −6.76 67 8 mv3 1 −29.78 160.24 −1.35 2 −34.38 185.43 27.65 3 −0.04 184.24 27.654 −0.04 159.21 −1.35 5 6 7 8 mv4 1 0.04 159.21 −1.35 2 0.04 184.24 27.653 34.38 185.43 27.65 4 29.78 160.24 −1.35 5 6 7 8 mv5 1 61.12 131.03−6.76 2 31.44 163.43 −6.76 3 33.41 174.24 5.74 4 43.75 183.72 25.78 577.92 146.42 25.78 6 7 8 mv6 1 74.79 88.94 −10.38 2 74.79 154.82 −10.383 114.09 154.82 16.17 4 114.09 88.94 16.17 5 6 7 8 mv7 1 −107.52 89.3530.99 2 −110.94 68.34 59.03 3 −136.32 120.65 95.14 4 −132.90 141.6667.10 5 6 7 8 mv8 1 −129.50 196.36 99.91 2 −139.66 144.56 68.88 3−133.18 126.69 96.58 4 −123.02 178.48 127.62 5 6 7 8 mv9 1 −42.26 185.7373.40 2 −65.99 163.92 49.03 3 −69.45 141.18 82.25 4 −45.72 162.99 106.625 6 7 8 mv10 1 0.00 190.18 78.00 2 −40.33 183.35 74.96 3 −46.98 168.27105.79 4 0.00 176.23 109.33 5 6 7 8 mv11 1 0.00 176.23 109.33 2 46.98168.27 105.79 3 40.33 183.35 74.96 4 0.00 190.18 78.00 5 6 7 8 mv12 142.26 185.73 73.40 2 45.72 162.99 106.62 3 69.45 141.18 82.25 4 65.99163.92 49.03 5 6 7 8 mv13 1 139.66 144.56 68.88 2 129.50 196.36 99.91 3123.02 178.48 127.62 4 133.18 126.69 96.58 5 6 7 8 mv14 1 132.90 141.6667.10 2 136.32 120.65 95.14 3 110.94 68.34 59.03 4 107.52 89.35 30.99 56 7 8 mv15 1 −59.72 111.27 102.01 2 −38.96 95.77 87.32 3 −42.25 116.9860.28 4 −79.46 144.76 86.61 5 −77.49 132.11 102.74 6 7 8 mv16 1 37.7388.59 93.83 2 29.22 119.90 64.12 3 −29.22 119.90 64.12 4 −37.73 88.5993.83 5 6 7 8 mv17 1 42.25 116.98 60.28 2 38.96 95.77 87.32 3 59.72111.27 102.01 4 79.46 144.76 86.61 5 42.25 116.98 60.28 6 7 8 mv18 1−63.87 149.13 93.46 2 −79.68 162.64 67.06 3 −100.06 208.14 102.55 4−84.26 194.63 128.95 5 6 7 8 mv19 1 −140.43 92.77 119.03 2 −140.43126.87 119.12 3 −136.72 174.44 128.44 4 −125.11 154.96 157.07 5 −130.4187.14 143.79 6 7 8 mv20 1 63.87 149.13 93.46 2 79.68 162.64 67.06 3100.06 208.14 102.55 4 84.26 194.63 128.95 5 6 7 8 mv21 1 130.41 87.14143.79 2 125.11 154.96 157.07 3 136.72 174.44 128.44 4 140.43 126.87119.12 5 140.43 92.77 119.03 6 7 8 mv22 1 −134.07 126.69 200.27 2−103.99 134.04 208.61 3 −94.62 209.63 108.20 4 −124.70 202.28 99.86 5 67 8 mv23 1 94.62 209.63 108.20 2 103.99 134.04 208.61 3 134.07 126.69200.27 4 124.70 202.28 99.86 5 6 7 8 mv24 1 −61.13 193.21 119.96 2−97.12 187.87 131.32 3 −97.12 169.38 170.59 4 −19.20 152.51 206.45 519.20 152.51 206.45 6 97.12 169.38 170.59 7 97.12 187.87 131.32 8 61.13193.21 119.96 mv25 1 −106.74 171.66 177.19 2 −83.23 85.77 217.46 3 0.0085.77 246.33 4 0.00 150.54 222.12 5 6 7 8 mv26 1 0.00 150.54 222.12 20.00 150.54 222.12 3 83.23 85.77 217.46 4 106.74 171.66 177.19 5 6 7 8

TABLE IV Scan Line Groups - Vertical Section Group Scanning Station/Identifier Mirrors in Group Scan Line Type gv1 mv1, mv22 VST1/4 verticalleft gv2 mv2, mv26 VST1/4 top-down vertical gv3 mv3, mv25 VST1/4top-down horizontal gv4 mv4, mv26 VST1/4 top-down horizontal gv5 mv5,mv25 VST1/4 top-down vertical gv6 mv6, mv23 VST1/4 vertical right gv7mv7, mv24 VST1/4 high horizontal left gv8 mv8, mv18, mv19 VST1/4 sidehorizontal left gv9 mv9, mv17, mv24 VST1/4 low horizontal left gv10mv10, mv16, mv26 VST1/4 top-down horizontal gv11 mv11, mv16, mv25 VST1/4top-down horizontal gv12 mv12, mv15, mv24 VST1/4 low horizontal rightgv13 mv13, mv20, mv21 VST1/4 side horizontal right gv14 mv14, mv24VST1/4 high horizontal right

In addition, as shown in FIG. 2F, the third laser scanning station VST1includes a light collecting/focusing optical element, e.g. paraboliclight collecting mirror or parabolic surface emulating volume reflectionhologram (labeled LC_(VST1)) that collects light from a scan region thatencompasses the outgoing scanning planes (produced by the third laserscanning station VST1) and focuses such collected light onto aphotodetector (labeled PD_(VST1)), which produces an electrical signalwhose amplitude is proportional to the intensity of light focusedthereon. The electrical signal produced by the photodetector is suppliedto analog/digital signal processing circuitry, associated with the thirdlaser scanning station VST1, that process analog and digital scan datasignals derived therefrom to perform bar code symbol reading operations.Preferably, the third laser scanning station VST1 includes a laser beamproduction module (not shown) that generates a laser scanning beam SB3that is directed to a small light directing mirror disposed in theinterior of the light collecting/focusing element LC_(VST1), whichredirects the laser scanning beam SB3 to a point of incidence on thesecond rotating polygonal mirror PM2.

In the illustrative embodiment, the first polygonal mirror PM1 includes4 facets that are used in conjunction with the two independent laserbeam sources provided by the first and second laser scanning stationsHST1 and HST2 so as project from the bottom-scanning window anomni-directional laser scanning pattern consisting of 40 laser scanningplanes that are cyclically generated as the first polygonal mirror PM1rotates. Moreover, the second polygonal mirror PM2 includes 4 facetsthat are used in conjunction with the independent laser beam sourceprovided by the third laser scanning station VST1 so as to project fromthe side-scanning window an omni-directional laser scanning patternconsisting of 28 laser scanning planes cyclically generated as thesecond polygonal mirror PM2 rotates. Thus, the bioptical laser scanningsystem of the illustrative embodiment project from the bottom andside-scanning windows an omni-directional laser scanning patternconsisting of 68 laser scanning planes cyclically generated as the firstand second polygonal mirrors PM1 and PM2 rotate. It is understood,however, these number may vary from embodiment to embodiment of thepresent invention and thus shall not form a limitation thereof.

FIG. 2G1 depicts the angle of each facet of the rotating polygonalmirrors PM1 and PM2 with respect to the rotational axis of therespective rotating polygonal mirrors in this illustrative embodiment.The scanning ray pattern produced by the four facets of the firstpolygonal mirror PM1 in conjunction with the laser beam source providedby the first laser scanning station HST1 in the illustrative embodimentis shown in FIG. 2G2. A similar scanning ray pattern is produced by thefour facets of the first polygonal mirror PM1 in conjunction with thelaser beam source provided by the second laser scanning station HST2. Inthe illustrative embodiment of the present invention, the secondrotating polygonal mirror PM2 has two different types of facets based onbeam elevation angle characteristics of the facet. The scanning raypattern produced by the four facets of the second polygonal mirror PM2in conjunction with the laser beam source provided by the third laserscanning station VST1 in the illustrative embodiment is shown in FIG.2G3. The facets of the second polygonal mirror PM2 can be partitionedinto two classes: a first class of facets (corresponding to angles β₁and β₂) have High Elevation (HE) angle characteristics, and a secondclass of facets (corresponding to angles β₃ and β₄) have Low Elevation(LE) angle characteristics. As shown in FIG. 2G2, high and low elevationangle characteristics are referenced by the plane P1 that contains theincoming laser beam and is normal to the rotational axis of the secondpolygonal mirror PM2. Each facet in the first class of facets (havinghigh beam elevation angle characteristics) produces an outgoing laserbeam that is directed above the plane P1 as the facet sweeps across thepoint of incidence of the third laser scanning station VST1. Whereaseach facet in the second class of facets (having low beam elevationangle characteristics) produces an outgoing laser beam that is directedbelow the plane P1 as the facet sweeps across the point of incidence ofthe third laser scanning station VST1. As will become apparenthereinafter, the use of scanning facets having such diverse elevationangle characteristics enables an efficient design and construction ofthe second section of the bioptical laser scanning—the plurality of beamfolding mirrors used therein can be compactly arranged within aminimized region of volumetric space. Such efficient space savingdesigns are advantageous in space-constricted POS-type scanningapplications.

In the illustrative embodiment of the present invention, the first laserscanning station (HST1) includes four groups of laser beam foldingmirrors (GH1, GH2, GH3, and GH4 as depicted in Table II above) which arearranged about the first rotating polygonal mirror PM1, and cooperatewith the four scanning facets of the first rotating polygonal mirror PM1so as to generate and project four different groups of laser scanningplanes (with 20 total scanning planes in the four groups) through thebottom-scanning window 16, as graphically illustrated in FIGS. 3A-3G.Note that the first laser scanning station HST1 utilizes mirrors MH4 andMH5 (and not MH6) of group GH4 to produce 8 different scan planestherefrom. The second laser scanning station (HST2) includes four groupsof laser beam folding mirrors (GH4, GH5, GH6 and GH7 as depicted inTable II) which are arranged about the first rotating polygonal mirrorPM1, and cooperate with the four scanning facets of the first rotatingpolygonal mirror so as to generate and project four different groups oflaser scanning planes (with 20 total scanning planes in the four groups)through the bottom-scanning window 16, as graphically illustrated inFIGS. 4A-4F. Note that the second laser scanning station HST2 utilizesmirrors MH5 and MH6 (and not MH4) of group GH4 to produce 8 differentscan planes therefrom. Finally, the third laser scanning station (VST1)includes fourteen groups of laser beam folding mirrors (GV1, GV2 . . .GV14 as depicted in Table IV above) which are arranged about the secondrotating polygonal mirror PM2, and cooperate with the four scanningfacets of the second rotating polygonal mirror PM2 so as to generate andproject fourteen different groups of laser scanning planes (with 28total scanning planes in the fourteen groups) through the side-scanningwindow 18, as graphically illustrated in FIGS. 5A-5Q.

For purposes of illustration and conciseness of description, each laserbeam folding mirror in each mirror group as depicted in the secondcolumn of Tables II and IV, respectively, is referred to in thesequential order that the outgoing laser beam reflects off the mirrorsduring the laser scanning plane generation process (e.g., the firstmirror in the column causes an outgoing laser beam to undergo its firstreflection after exiting a facet of the rotating polygonal mirror, thesecond mirror in the column causes the outgoing laser beam to undergoits second reflection, etc.).

First Laser Scanning Station HST1

As shown in FIGS. 2A, 2B and 3A-3F2, the first laser scanning station(HST1) includes four groups of laser beam folding mirrors (GH1, GH2, GH3and GH4) which are arranged about the first rotating polygonal mirrorPM1, and cooperate with the four scanning facets of the first rotatingpolygonal mirror PM1 so as to generate and project four different groupsof laser scanning planes (with 20 total scanning planes in the fourgroups) through the bottom-scanning window 16. The intersection of thefour groups of laser scanning planes (with 20 total scanning planes inthe four groups) on the bottom-scanning window 16 is shown in FIG. 3A.The twenty laser scanning planes (of these four groups projected throughthe bottom-scanning window 16) can be classified as either verticalscanning planes or horizontal scanning planes, which can be defined asfollows.

As shown in FIGS. 3B1 and 3B2, a scanning plane has a characteristicdirection of propagation D_(p) and a normal direction SP_(N), whichdefine a direction O that is orthogonal thereto (e.g., O=D_(p)×SP_(N)).For the sake of description, the characteristic direction of propagationD_(p) of a scanning plane can be defined as the mean propagationdirection for a plurality of rays that make up the scanning plane. Ahorizontal scanning plane is a scanning plane wherein the angle φbetween the direction O and the plane defined by the bottom-scanningwindow 16 is in the range between 0 and 45 degrees (and preferably inthe range between 0 and 20 degrees, and more preferably in the rangebetween 0 and 10 degrees). An exemplary horizontal scanning plane isshown in FIG. 3B1. A vertical scanning plane is a scanning plane whereinthe angle φ between the direction O and the plane defined by thebottom-scanning window 16 is in the range between 45 and 90 degrees (andpreferably in the range between 70 and 90 degrees, and more preferablyin the range between 80 and 90 degrees). An exemplary vertical scanningplane is shown in FIG. 3B2.

FIGS. 3C1 and 3C2 illustrate the first group GH1 of laser beam foldingmirrors of the first laser scanning station (HST1), which cooperate withthe four scanning facets of the first rotating polygonal mirror PM1 soas to generate four different vertical laser scanning planes thatproject from the right back corner of the bottom-scanning window 16diagonally outward and upward above the front left side (and front leftcorner) of the bottom-scanning window 16 as shown. These scanning planesare useful for reading ladder type bar code symbols disposed on bottom-,back-, and right-facing surfaces.

FIGS. 3D1 and 3D2 illustrate the second group GH2 of laser beam foldingmirrors of the first laser scanning station (HST1), which cooperate withthe four scanning facets of the first rotating polygonal mirror PM1 soas to generate four different horizontal laser scanning planes thatproject from the right side of the bottom-scanning window 16 diagonallyoutward and upward above the left side of the bottom-scanning window 16as shown. These scanning planes are useful for reading picket-fence typebar code symbols disposed on bottom- and right-facing surfaces.

FIGS. 3E1 and 3E2 illustrate the third group GH3 of laser beam foldingmirrors of the first laser scanning station (HST1), which cooperate withthe four scanning facets of the first rotating polygonal mirror PM1 soas to generate four different vertical laser scanning planes thatproject from the right front corner of the bottom-scanning window 16diagonally outward and upward above the back left side and back leftcorner of the bottom-scanning window 16 as shown. These scanning planesare useful for reading ladder type bar code symbols disposed on bottom-,front-, and right-facing surfaces.

FIGS. 3F1 and 3F2 illustrate the fourth group GH4 of laser beam foldingmirrors of the first laser scanning station (HST1), which cooperate withthe four scanning facets of the first rotating polygonal mirror PM1 soas to generate eight different horizontal laser scanning planes thatproject from the front side of the bottom-scanning window 16 diagonallyoutward and upward above the back side of the bottom-scanning window 16as shown. Note that the first laser scanning station HST1 utilizesmirrors MH4 and MH5 (and not MH6) of group GH4 to produce eightdifferent scan planes therefrom. These scanning planes are useful forreading picket-fence type bar code symbols disposed on bottom- andfront-facing surfaces.

The position and orientation of each beam folding mirror employed atscanning station HST1 relative to a global coordinate reference systemis specified by a set of position vectors pointing from the from theorigin of this global coordinate reference system to the vertices ofeach such beam folding mirror element (i.e. light reflective surfacepatch). The x,y,z coordinates of these vertex-specifying vectors as setforth above in Table I specify the perimetrical boundaries of these beamfolding mirrors employed in the scanning system of the illustrativeembodiment.

Second Laser Scanning Station HST2

As shown in FIGS. 2A, 2B and 4A-4E2, the second laser scanning station(HST2) includes four groups of laser beam folding mirrors (GH4, GH5,GH6, and GH7) which are arranged about the first rotating polygonalmirror PM1, and cooperate with the four scanning facets of the firstrotating polygonal mirror PM1 so as to generate and project fourdifferent groups of laser scanning planes (with 20 total scanning planesin the four groups) through the bottom-scanning window 16. Theintersection of the four groups of laser scanning planes (with 20 totalscanning planes in the four groups) on the bottom-scanning window 16 isshown in FIG. 4A. The twenty laser scanning planes (of these four groupsprojected through the bottom-scanning window 16) can be classified aseither vertical scanning planes or horizontal scanning planes as definedabove.

FIGS. 4B1 and 4B2 illustrate the first group (GH4) of laser beam foldingmirrors of the second laser scanning station (HST2), which cooperatewith the four scanning facets of the first rotating polygonal mirror PM1so as to generate eight different horizontal laser scanning planes thatproject from the front side of the bottom-scanning window 16 diagonallyoutward and upward above the back side of the bottom-scanning window 16as shown. Note that the second laser scanning station HST2 utilizesmirrors MH5 and MH6 (and not MH4) of group GH4 to produce eightdifferent scan planes therefrom. These scanning planes are useful forreading picket-fence type bar code symbols disposed on bottom- andfront-facing surfaces.

FIGS. 4C1 and 4C2 illustrate the second group (GH5) of laser beamfolding mirrors of the second laser scanning station (HST2), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the left front corner of the bottom-scanningwindow 16 diagonally outward and upward above the back right side andback right corner of the bottom-scanning window 16 as shown. Thesescanning planes are useful for reading ladder type bar code symbolsdisposed on bottom-, front-, and left-facing surfaces.

FIGS. 4D1 and 4D2 illustrate the third group (GH6) of laser beam foldingmirrors of the second laser scanning station (HST2), which cooperatewith the four scanning facets of the first rotating polygonal mirror PM1so as to generate four different horizontal laser scanning planes thatproject from the left side of the bottom-scanning window 16 diagonallyoutward and upward above the right side of the bottom-scanning window 16as shown. These scanning planes are useful for reading picket-fence typebar code symbols disposed on bottom- and left-facing surfaces.

FIGS. 4E1 and 4E2 illustrate the fourth group (GH7) of laser beamfolding mirrors of the second laser scanning station (HST2), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the left back corner of the bottom-scanningwindow 16 diagonally outward and upward above the front right side andfront right corner of the bottom-scanning window 16 as shown. Thesescanning planes are useful for reading ladder type bar code symbolsdisposed on bottom-, back-, and left-facing surfaces.

The position and orientation of each beam folding mirror employed atscanning station HST2 relative to a global coordinate reference systemis specified by a set of position vectors pointing from the from theorigin of this global coordinate reference system to the vertices ofeach such beam folding mirror element (i.e. light reflective surfacepatch). The x,y,z coordinates of these vertex-specifying vectors as setforth above in Table I specify the perimetrical boundaries of these beamfolding mirrors employed in the scanning system of the illustrativeembodiment.

As shown in FIG. 4F, the vertical scanning planes that project from thebottom-scanning window 16 include 4 groups (namely, GH1, GH3, GH5 andGH7). Groups GH1 and GH5 project from opposing portions (e.g., theback-right and front-left corners of the window 16) of thebottom-scanning window 16, and groups GH3 and GH7 project from opposingportions (e.g., front-right and back-left corners of the window 16) ofthe bottom-scanning window. Note that groups GH1 and GH5 aresubstantially co-planar (i.e., quasi co-planar) and groups GH3 and GH7are substantially co-planar (i.e., quasi co-planar), while groups GH1and GH5 are substantially orthogonal (i.e., quasi-orthogonal) to groupsGH3 and GH7, respectively, as shown.

Third Laser Scanning Station VST1

As shown in FIGS. 2D, 2E and 5A-5P2, the third laser scanning station(VST1) includes fourteen groups of laser beam folding mirrors (GV1, GV2,GV3 . . . GV14) which are arranged about the second rotating polygonalmirror PM2, and cooperate with the four scanning facets of the secondrotating polygonal mirror PM2 so as to generate and project fourteendifferent groups of laser scanning planes (with 28 total scanning planesin the fourteen groups) through the side-scanning window 18. Theintersection of the fourteen groups of laser scanning planes (with 28total scanning planes in the fourteen groups) on the side-scanningwindow 18 is shown in FIG. 5A. The twenty-eight laser scanning planes(of these fourteen groups projected through the side-scanning window 18)can be classified as either vertical scanning planes or horizontalscanning planes, which can be defined as follows.

As shown in FIGS. 5B1 and 5B2, a scanning plane has a characteristicdirection of propagation D_(p) and a normal direction SP_(N), whichdefine a direction O that is orthogonal thereto (e.g., O=D_(p)×SP_(N)).A horizontal scanning plane is a scanning plane wherein the angle φbetween the direction O and the plane defined by the bottom-scanningwindow 16 is in the range between 0 and 45 degrees (and preferably inthe range between 0 and 20 degrees, and more preferably in the rangebetween 0 and 10 degrees). An exemplary horizontal scanning planeprojected from the side-scanning window 18 is shown in FIG. 5B1. Avertical scanning plane is a scanning plane wherein the angle φ betweenthe direction O and the plane defined by the bottom-scanning window 16is in the range between 45 and 90 degrees (and preferably in the rangebetween 70 and 90 degrees, and more preferably in the range between 80and 90 degrees). An exemplary vertical scanning plane projected from theside-scanning window 18 is shown in FIG. 5B2.

FIGS. 5C1 and 5C2 illustrate the first group (GV1) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two low-elevation (LE) scanning facets of the second rotatingpolygonal mirror PM2 (corresponding to angles β₃ and β₄ of the secondpolygonal mirror PM2 in FIG. 2G1) so as to generate two differentvertical laser scanning planes that project from the left side of theside-scanning window 18 diagonally down and out across thebottom-scanning window 16 above the front right corner of thebottom-scanning window 16 as shown. These scanning planes are useful forreading ladder type bar code symbols disposed on left- and back-facingsurfaces.

FIGS. 5D1 and 5D2 illustrate the second group (GV2) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low-elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differentvertical laser scanning planes that project from the top left corner ofthe side-scanning window 18 downward toward the bottom-scanning window16 substantially along the left side of the bottom-scanning window 16 asshown. These scanning planes are useful for reading ladder type bar codesymbols disposed on top- and back-facing surfaces.

FIGS. 5E1 and 5E2 illustrate the third group (GV3) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two low-elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₃ and β₄ of the second polygonalmirror PM2 in FIG. 2G1) so as to generate two different horizontal laserscanning planes that project from the top left quadrant of theside-scanning window 18 diagonally down across the bottom-scanningwindow 16 as shown. These scanning planes are useful for readingpicket-fence type bar code symbols disposed on back- and top-facingsurfaces.

FIGS. 5F1 and 5F2 illustrate the fourth group (GV4) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the top rightquadrant of the side-scanning window 18 diagonally down across thebottom-scanning window 16 as shown. These scanning planes are useful forreading picket-fence type bar code symbols disposed on back- andtop-facing surfaces.

FIGS. 5G1 and 5G2 illustrate the fifth group (GV5) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two low-elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₃ and β₄ of the second polygonalmirror PM2 in FIG. 2G1) so as to generate two different vertical laserscanning planes that project from the top right corner of theside-scanning window 18 downward toward the bottom-scanning window 16substantially along the right side of the bottom-scanning window 16 asshown. These scanning planes are useful for reading ladder type bar codesymbols disposed on top- and back-facing surfaces.

FIGS. 5H1 and 5H2 illustrate the sixth group (GV6) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two low elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₃ and β₄ of the second polygonalmirror PM2 in FIG. 2G1) so as to generate two different vertical laserscanning planes that project from the right side of the side-scanningwindow 18 diagonally out across the bottom-scanning window 16 above thefront left corner of the bottom-scanning window 16 as shown. Thesescanning planes are useful for reading ladder type bar code symbolsdisposed on right- and back-facing surfaces.

FIGS. 5I1 and 5I2 illustrate the seventh group (GV7) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the top left quadrantof the side-scanning window 18 outwardly across the bottom-scanningwindow 16 (substantially parallel to the bottom-scanning window 16) asshown. These scanning planes are useful for reading picket-fence typebar code symbols disposed on back- and left-facing surfaces.

FIGS. 5J1 and 5J2 illustrate the eight group (GV8) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two high elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₁ and β₂ of the second polygonalmirror PM2 in FIG. 2G1) so as to generate two different horizontal laserscanning planes that project from the left side of the side-scanningwindow 18 outwardly across the bottom-scanning window 16 (substantiallyparallel to the bottom-scanning window 16) as shown. In the illustrativeembodiment, the characteristic direction of propagation of such scanningplanes has a non-vertical component (i.e., components in the planeparallel to the bottom-scanning window 16) whose orientation relative tothe normal of the side-scanning window 18 is greater than 35 degrees.These scanning planes are useful for reading picket-fence type bar codesymbols disposed on back- and left-facing surfaces (including thosesurfaces whose normals are substantially offset from the normal of theside-scanning window).

FIGS. 5K1 and 5K2 illustrate the ninth group (GV9) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two high elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₁ and β₂ of the second polygonalmirror PM2 in FIG. 2G1) so as to generate two different horizontal laserscanning planes that project from the central portion of theside-scanning window 18 outwardly and downward across thebottom-scanning window 16 as shown. These scanning planes are useful forreading picket-fence type bar code symbols disposed on back-facingsurfaces.

FIGS. 5L1 and 5L2 illustrate the tenth group (GV10) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the central portionof the side-scanning window 18 outwardly and sharply downward across thebottom-scanning window 16 as shown. These scanning planes are useful forreading picket-fence type bar code symbols disposed on top- andback-facing surfaces.

FIGS. 5M1 and 5M2 illustrate the eleventh group (GV11) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the central portionof the side-scanning window 18 outwardly and sharply downward across thebottom-scanning window 16 as shown. These scanning planes are useful forreading picket-fence type bar code symbols disposed on top- andback-facing surfaces.

FIGS. 5N1 and 5N2 illustrate the twelfth group (GV12) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the central portionof the side-scanning window 18 outwardly across the bottom-scanningwindow 16 (substantially parallel to the bottom-scanning window 16) asshown. These scanning planes are useful for reading picket-fence typebar code symbols disposed on back-facing surfaces.

FIGS. 5O1 and 5O2 illustrate the thirteenth group (GV13) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the right side of theside-scanning window 18 outwardly across the bottom-scanning window 16(substantially parallel to the bottom-scanning window 16) as shown. Inthe illustrative embodiment, the characteristic direction of propagationof such scanning planes has a non-vertical component (i.e., componentsin the plane parallel to the bottom-scanning window 16) whoseorientation relative to the normal of the side-scanning window 18 isgreater than 35 degrees. These scanning planes are useful for readingpicket-fence type bar code symbols disposed on back- and right-facingsurfaces (including those surfaces whose normals are substantiallyoffset from the normal of the side-scanning window).

FIGS. 5P1 and 5P2 illustrate the fourteenth group (GV14) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2G1) so as to generate two differenthorizontal laser scanning planes that project from the top rightquadrant of the side-scanning window 18 outwardly across thebottom-scanning window 16 (substantially parallel to the bottom-scanningwindow 16) as shown. These scanning planes are useful for readingpicket-fence type bar code symbols disposed on back- and right-facingsurfaces.

The position and orientation of each beam folding mirror employed atscanning station VST1 relative to a global coordinate reference systemis specified by a set of position vectors pointing from the from theorigin of this global coordinate reference system to the vertices ofeach such beam folding mirror element (i.e. light reflective surfacepatch). The x,y,z coordinates of these vertex-specifying vectors as setforth above in Table III specifies the perimetrical boundaries of thesebeam folding mirrors employed in the scanning system of the illustrativeembodiment.

In the illustrative embodiment of the present invention, the laser beamfolding mirrors associated with scanning stations HST1, HST2 and VST1are physically supported utilizing one or more mirror support platforms,formed with the scanner housing. Preferably, these mirror mountingsupport structures, as well as the components of the scanning housingare made from a high-impact plastic using injection molding techniqueswell known in the art.

In the scanning system of the present invention, the principal functionof each facet on the first and second rotating polygonal mirrors PM1 andPM2 is to deflect an incident laser beam along a particular path in 3-Dspace in order to generate a corresponding scanning plane within the 3-Dlaser scanning volume produced by the laser scanning system hereof.Collectively, the complex of laser scanning planes produced by theplurality of facets in cooperation with the three laser beam productionmodules of HST1, HST2 and VST1 creates an omni-directional scanningpattern within the highly-defined 3-D scanning volume of the scanningsystem between the space occupied by the bottom and side-scanningwindows of the system. As shown in the exemplary timing scheme of FIG.6, the bioptical laser scanner of the illustrative embodiment cyclicallygenerates a complex omni-directional 3-D laser scanning pattern fromboth the bottom and side-scanning windows 16 and 18 thereof during therevolutions of the scanning polygonal mirrors PM1 and PM2. In thisexemplary timing scheme, four sets of scan plane groups (4*[GH1 . . .GH7]) are produced by stations HST1 and HST2 during each revolution ofthe polygonal mirror PM1 concurrently with a two sets of scan planegroups (2*[GV1. GV14]) produced by station VST1 during a singlerevolution of the polygonal mirror PM2. This complex omni-directionalscanning pattern is graphically illustrated in FIGS. 3A through 5P2. The3-D laser scanning pattern of the illustrative embodiment consists of 68different laser scanning planes, which cooperate in order to generate aplurality of quasi-orthogonal laser scanning patterns within the 3-Dscanning volume of the system, thereby enabling true omnidirectionalscanning of bar code symbols.

In each laser scanning station (HST1, HST2, and VST1) of theillustrative embodiment, a laser beam production module produces a laserbeam that is directed at the point of incidence on the facets of thefirst or second rotating polygonal mirrors at the pre-specified angle ofincidence. Preferably, such laser beam production modules comprises avisible laser diode (VLD) and possibly an aspheric collimating lenssupported within the bore of a housing mounted upon the optical bench ofthe module housing.

In the illustrative embodiment described above, the pre-specified angleof incidence of the laser beams produced by the laser beam productionmodules of the laser scanning stations HST1 and HST2 are offset from therotational axis of the polygonal mirror PM1 along a directionperpendicular to the rotational axis as shown in FIG. 2H. Such offsetprovides for spatial overlap in the scanning pattern of light beamsproduced from the polygonal mirror PM1 by these laser beam productionmodules. In the illustrative embodiment, the offset between therotational axis of the rotating polygonal mirror PM1 and the incidentdirections of the scanning beams SB1 and SB2, respectively, isapproximately 5 mm. Such spatial overlap can be exploited such that theoverlapping rays are incident on at least one common mirror (mh5 in theillustrative embodiment) to provide a dense scanning pattern projectingtherefrom. In the illustrative embodiment, a dense pattern of horizontalplanes (groups GH4) is projected from the front side of the bottomwindow as is graphically depicted in FIGS. 3F1, 3F2 and 4B1 and 4B2.

Light Collection for the 3 Scanning Stations

When a bar code symbol is scanned by any one of the laser scanningplanes projected from the bottom-scanning window 16 (by either the firstor second laser scanning stations HST1, HST2), or by any one of thelaser scanning planes projected from the side-scanning window 18 by thethird laser scanning station VST1, the incident laser light scannedacross the object is intensity modulated by the absorptive properties ofthe scanned object and scattered according to Lambert's Law (for diffusereflective surfaces). A portion of this laser light is reflected backalong the outgoing ray (optical) path, off the same group of beamfolding mirrors employed during the corresponding laser beam generationprocess, and thereafter is incident on the same scanning facet (of thefirst or second rotating polygonal mirror) that generated thecorresponding scanning plane only a short time before. The scanningfacet directs the returning reflected laser light towards a lightcollecting optical element (e.g., parabolic mirror structure) of therespective laser scanning station, which collects the returning lightand focuses these collected light rays onto a photodetector, which maybe disposed on a planar surface beneath the respective scanning polygon(as shown in FIGS. 2C1 and 2C2), or which may be disposed on a planarsurface above the respective scanning polygon (as shown in FIG. 2F).FIGS. 2C1 and 2C2 depict the light collection optical elements LC_(HST1)and LC_(HST2), e.g., parabolic mirrors, and photodetectors PD_(HST1) andPD_(HST2) for the two laser scanning stations HST1 and HST2,respectively. FIG. 2F depicts the light collection optical elementsLC_(VST1), e.g., parabolic mirror, and photodetector PD_(VST1) for thethird laser scanning station VST1. The electrical signal produced by thephotodetector for the respective laser scanning stations is supplied toanalog/digital signal processing circuitry, associated with therespective laser scanning stations, that process analog and digital scandata signals derived therefrom to perform bar code symbol readingoperations.

As shown in FIG. 1A, the bottom and side-scanning windows 16 and 18 havelight transmission apertures of substantially planar extent. In order toseal off the optical components of the scanning system from dust,moisture and the like, the scanning windows 16 and 18, are preferablyfabricated from a high impact plastic material and installed over theircorresponding light transmission apertures using a rubber gasket andconventional mounting techniques. In the illustrative embodiment, eachscanning window 16 and 18 preferably has spectrally-selective lighttransmission characteristics which, in conjunction with aspectrally-selective filters installed before each photodetector withinthe housing, forms a narrow-band spectral filtering subsystem thatperforms two different functions. The first function of the narrow-bandspectral filtering subsystem is to transmit only the optical wavelengthsin the red region of the visible spectrum in order to impart a reddishcolor or semi-transparent character to the scanning window. This makesthe internal optical components less visible and thus remarkablyimproves the external appearance of the bioptical laser scanning system.This feature also makes the bioptical laser scanner less intimidating tocustomers at point-of-sale (POS) stations where it may be used. Thesecond function of the narrow-band spectral filtering subsystem is totransmit to the photodetector for detection, only the narrow band ofspectral components comprising the outgoing laser beam produced by theassociated laser beam production module. Details regarding this opticalfiltering subsystem are disclosed in copending application Ser. No.08/439,224, entitled “Laser Bar Code Symbol Scanner Employing OpticalFiltering With Narrow Band-Pass Characteristics and Spatially SeparatedOptical Filter Elements” filed on May 11, 1995, which is incorporatedherein by reference in its entirety.

Electrical Subsystem

In the illustrative embodiment of the present invention, the biopticallaser scanning system 1 comprises a number of system components as shownin the system diagram of FIG. 7, including: photodetectors (e.g. asilicon photocell) for detection of optical scan data signals generatedby the respective laser scanning stations; analog signal processingcircuitry for processing (e.g., preamplification, bandpass filtering,and A/D conversion) analog scan data signals, digitizing circuitry forconverting the digital scan data signal D₂, associated with each scannedbar code symbol, into a corresponding sequence of digital words (i.e. asequence of digital count values) D₃, and bar code symbol decodingcircuitry that receives the digital word sequences D₃ produced from thedigitizing circuit, and subject it to one or more bar code symboldecoding algorithms in order to determine which bar code symbol isindicated (i.e. represented) by the digital word sequence D₃.

As described above, during laser scanning operations, the optical scandata signal D₀ focused onto the photodetectors 45A 45B, and 45C isproduced by light rays associated with a diffracted laser beam beingscanned across a light reflective surface (e.g. the bars and spaces of abar code symbol) and scattering thereof, whereupon the polarizationstate distribution of the scattered light rays is typically altered whenthe scanned surface exhibits diffuse reflective characteristics.Thereafter, a portion of the scattered light rays are reflected alongthe same outgoing light ray paths toward the facet which produced thescanned laser beam. These reflected light rays are collected by thescanning facet and ultimately focused onto the photodetector by itsparabolic light reflecting mirror. The function of each photodetector isto detect variations in the amplitude (i.e. intensity) of optical scandata signal D₀, and produce in response thereto an electrical analogscan data signal D₁ which corresponds to such intensity variations. Whena photodetector with suitable light sensitivity characteristics is used,the amplitude variations of electrical analog scan data signal D₁ willlinearly correspond to light reflection characteristics of the scannedsurface (e.g. the scanned bar code symbol). The function of the analogsignal processing circuitry is to amplify and band-pass filter theelectrical analog scan data signal D₁, in order to improve the SNR ofthe analog signal, and convert the analog signal into digital form(e.g., a pulse train with transitions between high and low logiclevels). In practice, this analog to digital conversion is athresholding function which converts the electrical analog scan datasignal D₁ into a corresponding digital scan data signal D₂ having firstand second (i.e. binary) signal levels which correspond to the bars andspaces of the bar code symbol being scanned. Thus, the digital scan datasignal D₂ appears as a pulse-width modulated type signal as the firstand second signal levels vary in proportion to the width of bars andspaces in the scanned bar code symbol.

The digitizing circuitry converts the digital scan data signal D₂,associated with each scanned bar code symbol, into a correspondingsequence of digital words (i.e. a sequence of digital count values) D₃.Notably, in the digital word sequence D₃, each digital word representsthe time length associated with each first or second signal level in thecorresponding digital scan data signal D₂. Preferably, these digitalcount values are in a suitable digital format for use in carrying outvarious symbol decoding operations which, like the scanning pattern andvolume of the present invention, will be determined primarily by theparticular scanning application at hand. Reference is made to U.S. Pat.No. 5,343,027 to Knowles, incorporated herein by reference, as itprovides technical details regarding the design and construction ofmicroelectronic digitizing circuits suitable for use in the laserscanner of the present invention.

The bar code symbol decoding circuitry receive each digital wordsequence D₃ produced from the digitizing circuit, and subject it to oneor more bar code symbol decoding algorithms in order to determine whichbar code symbol is indicated (i.e. represented) by the digital wordsequence D₃, originally derived from corresponding scan data signal D₁detected by the photodetector associated therewith. In more generalscanning applications, the function of the programmed decode computer isto receive each digital word sequence D₃ produced from the digitizingcircuit, and subject it to one or more pattern recognition algorithms(e.g. character recognition algorithms) in order to determine whichpattern is indicated by the digital word sequence D₃. In bar code symbolreading applications, in which scanned code symbols can be any one of anumber of symbologies, a bar code symbol decoding algorithm withauto-discrimination capabilities can be used in a manner known in theart.

As shown in FIG. 7, the system includes a programmed microprocessor 61with a system bus and associated program and data storage memory, forcontrolling the system operation of the bioptical laser scanner andperforming other auxiliary functions and for receiving bar code symbolcharacter data (provided by the bar code symbol decoding circuitry); adata transmission subsystem for interfacing with and transmitting symbolcharacter data and other information to host computer system (e.g.central computer, cash register, etc.) over a communication linktherebetween; and an input/output interface for providing drive signalsto an audio-transducer and/or LED-based visual indicators used to signalsuccessful symbol reading operations to users and the like, forproviding user input via interaction with a keypad, and for interfacingwith a plurality of accessory devices (such as an external handheldscanner that transmits bar code symbol character data to the biopticallaser scanning system, a display device, a weight scale, a magnetic cardreader and/or a coupon printer as shown). In addition, the input-outputinterface may provide a port that enables an external handheld scannerto transmit sequences of digital words D₃ (i.e. a sequence of digitalcount values) generated therein to the bioptical laser scanning systemfor bar code symbol decoding operations. Details of such an interfaceport are described in U.S. Pat. No. 5,686,717 to Knowles et al.,commonly assigned to the assignee of the present invention, hereinincorporated by reference in its entirety.

The communication link between the data transmission subsystem and thehost system may be a wireless data link (such as an infra-red link,Bluetooth RF link or IEEE 802.11a or 802.11b RF link) or wired serialdata link (such as keyboard wedge link—for example supporting XT-, AT-and PS/2-style keyboard protocols, an RS-232 link, USB link, a Firewire(or IEEE 1394) link, an RS-422 link, and RS-485 link), a wired paralleldata bus, or other common wired interface links (such as an OCIA link,IBM 46XX link, Light Pen Emulation link, LTPN link). Similarly, theinput/output interface between the external handheld scanner and thebioptical laser scanning system may support a wireless data link (suchas an infra-red link, Bluetooth RF link or IEEE 802.11a or 802.11b RFlink) or wired serial data link (such as keyboard wedge link—for examplesupporting XT-, AT- and PS/2-style keyboard protocols, an RS-232 link,USB link, a Firewire (or IEEE 1394) link, an RS-422 link, and RS-485link), a wired parallel data bus, or other common wired interface links(such as an OCIA link, IBM 46XX link, Light Pen Emulation link, LTPNlink).

The microprocessor also produces motor control signals, and lasercontrol signals during system operation. These control signals as wellas a 120 Volt, 60 Hz line voltage signal from an external power source(such as a standard power distribution circuit) are received as input bya power regulation circuit, which produces as output, (1) laser sourceenable signals to drive VLDs 153A, 153B, 153C, and 153D, respectively,and (2) motor enable signals in order to drive the two motors (motor 1and motor 2) that cause rotation of the first and second rotatingpolygonal mirrors, respectively.

In some scanning applications, where omni-directional scanning cannot beensured at all regions within a pre-specified scanning volume, it may beuseful to use scan data produced either (i) from the same laser scanningplane reproduced many times over a very short time duration while thecode symbol is being scanned therethrough, or (ii) from severaldifferent scanning planes spatially contiguous within a pre-specifiedportion of the scanning volume. In the first instance, if the bar codesymbol is moved through a partial region of the scanning volume, anumber of partial scan data signal fragments associated with the movedbar code symbol can be acquired by a particular scanning plane beingcyclically generated over an ultra-short period of time (e.g. 1-3milliseconds), thereby providing sufficient scan data to read the barcode symbol. In the second instance, if the bar code symbol is withinthe scanning volume, a number of partial scan data signal fragmentsassociated with the bar code symbol can be acquired by several differentscanning planes being simultaneously generated by the three laserscanning stations of the system hereof, thereby providing sufficientscan data to read the bar code symbol, that is, provided such scan datacan be identified and collectively gathered at a particular decodeprocessor for symbol decoding operations.

In order to allow the bioptical scanning system of the present inventionto use symbol decoding algorithms that operate upon partial scan datasignal fragments, as described above, a synchronizing signal can be usedto identify a set of digital word sequences D₃, (i.e. {D_(S)}),associated with a set of time-sequentially generated laser scanningbeams produced by a particular facet on the first and second rotatingpolygonal mirrors. In such applications, each set of digital wordsequences can be used to decode a partially scanned code symbol andproduce symbol character data representative of the scanned code symbol.In code symbol reading applications where complete scan data signals areused to decode scanned code symbols, the synchronizing signal describedabove need not be used, as the digital word sequence D₃ corresponding tothe completely scanned bar code symbol is sufficient to carry out symboldecoding operations using conventional symbol decoding algorithms knownin the art.

The synchronizing signal can be derived from a position sensor (such asa hall sensor), integrated into the rotating shaft (or other portion) ofthe rotating polygonal mirror, that generates an electrical signal whenthe rotating polygonal mirror reaches a predetermined point (such as astart-of-scan position) in its rotation. Alternatively, suchsynchronization may be derived from a position indicating opticalelement (e.g., mirror or lens), which is preferably mounted adjacent (ornear) the perimeter of one of the light folding mirrors, such that theposition indicating optical element is illuminated by the scanning beamwhen the rotating polygonal mirror reaches a predetermined point (suchas a start-of-scan position) in its rotation. The position indicatingoptical element may be a mirror that directs the illumination of thescanning beam incident thereon to a position indicating optical detector(which generates an electrical signal whose amplitude corresponds to theintensity of light incident thereon). Alternatively, the positionindicating optical element may be a light collecting lens that isoperably coupled to a light guide (such as a fiber optic bundle) thatdirects the illumination of the scanning beam incident thereon to aposition indicating optical detector (which generates an electricalsignal whose amplitude corresponds to the intensity of light incidentthereon).

As each synchronizing pulse in the synchronizing signal is synchronouswith a “reference” point on the respective rotating mirror, the symboldecoding circuitry provided with this periodic signal can readily “linkup” or relate, on a real-time basis, such partial scan data signalfragments with the particular facet on the respective rotating polygonalmirror that generated the partial scan data fragment. By producing botha scan data signal and a synchronizing signal as described above, thebioptical laser scanning system of the present invention can readilycarry out a diverse repertoire of symbol decoding processes which usepartial scan data signal fragments during the symbol reading process.

Modifications

The bioptical laser scanning system of the present invention can bemodified in various ways. For example, more (or less) groups of beamfolding mirrors can be used in each laser scanning station within thesystem and/or more or less facets can be used for the rotating polygonalmirrors, Such modifications will add (or remove) scanning planes fromthe system.

Also more or less laser scanning stations might be employed within thesystem. Such modifications might be practiced in order to provide anomnidirectional laser scanning pattern having scanning performancecharacteristics optimized for a specialized scanning application.

While the second rotating polygonal mirror of the illustrativeembodiment employs facets having low and high elevation anglecharacteristics, it is understood that it might be desirable inparticular applications to use scanning facets with differentcharacteristics (such as varying angular reflection characteristics) soas to enable a compact scanner design in a particular application.

Also, it is contemplated that each laser scanning station may not haveits own laser source (e.g., VLD). More specifically, as is well known inthe scanning art, the laser light produced by a laser source (VLD) maybe split into multiple beams (with a beam splitter) and directed tomultiple laser scanning stations with mirrors, a light pipe or otherlight directing optical element.

While the various embodiments of the bioptical laser scanner hereof havebeen described in connection with linear (1-D) bar code symbol scanningapplications, it should be clear, however, that the scanning apparatusand methods of the present invention are equally suited for scanning 2-Dbar code symbols, as well as alphanumeric characters (e.g. textualinformation) in optical character recognition (OCR) applications, aswell as scanning graphical images in graphical scanning arts.

Improved Scan Signal Processing

In any laser scanning system (including the various embodiments of thebioptical laser scanner described above), the primary function of thelaser scanning mechanism is to produce a laser scanning field (orvolume) in which bar code symbols can be scanned in a reliable manner.In such systems, the speed of the laser beam spot (or cross-section)along the extent of the scanned laser beam will vary over the depth ofthe scanning range of the system. The further the laser beam spot isaway from the laser scanning mechanism, the greater the laser beam spotspeed with be, based on well known principles of physics. A usefulmeasure of such beam spot speed variation is given by the ratio of (i)the maximum laser beam spot speed within the scanning field of thesystem, to (ii) the minimum laser beam spot speed in the scanningsystem. Hereinafter, this spot speed variation measure shall be referredto as the “Max/Min Beam Spot Speed Ratio” of a laser scanning system.

The substrate, usually paper, on which a bar code is printed reflects asignal of varying power when scanned with a focused laser beam within agiven focal zone in the system. The laser light energy reflected (i.e.scattered) off the scanned code symbol is directed onto a photodetectorby way of light collection and focusing optics. The photodetectorconverts these optical signals into corresponding electrical signals.The signal components produced by scanning the bar code substrate areunwanted and therefore are described as noise. Since the substrate isusually paper, consisting of fibers having a random spatial structure,such unwanted noise signals are commonly referred to as paper orsubstrate noise. A signal derived from the output of the photodetector(in analog or digital form) is referred to as a scan data signalS_(analog) comprising the desired bar code signal component as well asthe paper noise components.

As a bar code is scanned within a focal zone disposed further away fromthe scanner, the scan data signal is increasingly compressed on thetime-domain by virtue of the fact that the laser beam speed increases asa function of distance away from the laser scanning mechanism. Inaccordance with Fourier Analysis principles, compression of the scandata signal (including its noise components) represented on thetime-domain results in an increase in or shift of power to the higherspectral components of the scan data signal represented on thefrequency-domain. Thus, the frequency spectra of the scan data signal(including its noise components) undergoes a positive frequency shift asthe corresponding bar code symbol is scanned further away from the laserscanning system. This phenomenon is graphically illustrated in theanalog scan data signal of FIGS. 8A and 8B.

When scanning bar code symbols in a multi-focal zone laser scanningsystem, filters and signal thresholding devices are useful for rejectingnoise components in the scan data signal. However, such devices alsolimit the scan resolution of the system, potentially rendering thesystem incapable of reading low contrast and high resolution bar codesymbols on surfaces placed in the scanning field. Thus, it is imperativethat the bandwidth of the system be sufficient to support the spectralcomponents of scan data signals at different focal zones of the systemand to support the scanning of the desired resolution of bar codesymbols on surfaces placed in the scanning field.

In accordance with teachings of the present invention, a laser scanningsystem (such as the bioptical laser scanning system of the presentinvention as described above) includes a multi-path scan data signalprocessor having multiple signal processing paths. Each signalprocessing path processes the same data signal (which is derived fromthe output of a photodetector) to detect bar code symbols therein andgenerate data representing the bar code symbols. And each signalprocessing path has different operational characteristics (such aslow-pass filter cutoff frequencies, amplifier gain characteristics,and/or positive and negative signal thresholds). The varying operationalcharacteristics of the paths are optimized to provide different signalprocessing functions (e.g., minimize paper noise, or maximize the scanresolution of the system). The data signal derived from laser scanningis supplied to each path of the multi-path scan data processor, where itis processed (preferably in parallel) to identify signal leveltransitions therein. A digital scan data signal that encodes such signallevel transitions is provided to digitizing circuitry, which convertsthe digital scan data signal into a corresponding sequence of digitalwords (i.e. a sequence of digital count values) suitable for bar codesymbol decoding as described above.

By virtue of the present invention, it is now possible to identifysignal level transitions in the scan data signal over a diverse range ofoperating conditions (e.g., operating conditions where paper noise ispresent in addition to operating conditions requiring high resolutionscanning, such as the reading of low contrast or high resolution barcode symbols), which enables more reliable bar code reading over suchdiverse operating conditions. These and other advantages of the presentinvention will become apparent hereinafter.

Analog Scan Data Signal Processor of the Illustrative Embodiment of thePresent Invention

As shown in FIG. 9, a multi-path scan data signal processor 901according to the present invention comprises a number of subcomponents,namely: signal conditioning circuitry 903 operably coupled between aphotodetector 902 and a plurality of signal processing paths (two shownas path A and path B) that process the output of the signal conditioningcircuitry in parallel. Each signal processing path includes: a firstderivative signal generation circuit 904 having a differentiator, lowpass filter and amplifier therein; a second derivative signal generationcircuit 906 having a differentiator therein; a first derivative signalthreshold-level generation circuit 905; and a zero crossing detector907, data gate 908, and binary-type A/D signal conversion circuitry 909.

The signal conditioning circuitry 903 operates to smooth out orotherwise filter the scan data signal produced by the photodetector 902to remove unwanted noise components therein, and possibly amplify suchsignal. An illustrative implementation of such signal conditioningcircuitry is described below with respect to FIGS. 11A and 11B. Theoutput of the signal conditioning circuitry 903 is provided to theplurality of signal processing paths (two shown as path A and path B)that process the output of the signal conditioning circuitry 903 inparallel.

The first derivative signal generation circuitry 904 in each respectivepath (labeled 904-A and 904-B in as shown) includes a differentiator,low pass filter and amplifier that generate a signal approximating thefirst derivative of the analog scan data signal (with unwanted noisecomponents removed). The low pass filter may be implemented with passiveelements (such as resistors, capacitors and inductors) or may beimplemented with active elements (such as an operational amplifier).Preferably, the low-pass filter implements one of a Butterworth-type,Chebsychev-type, MFTD-type, or elliptical-type low pass filteringtransfer function, which are well known in the filtering art. Details ofthe design of such filters is set forth in the book entitled “ElectricalFilter Design Handbook,” Third Edition, by A. Williams et al.,McGraw-Hill, 1996, herein incorporated by reference in its entirety. Anillustrative implementation of the first derivative signal generationcircuitry 904 for two different paths is described below with respect toFIG. 12.

The “first derivative signal” is supplied to second derivative signalgeneration circuit 906 and to first derivative threshold circuitry 905in the respective path. The second derivative signal generationcircuitry in each respective path (labeled 906-A and 906-B as shown)includes a differentiator that generates a signal approximating thesecond derivative of the scan data signal (with unwanted noisecomponents removed). An example of the second derivative signalgeneration circuitry is described below with respect to FIG. 13.

The “second derivative signal” is supplied to a zero crossing detector907 that produces output signal(s) (“zero crossing signal”) identifyingzero crossings in the second derivative signal. An illustrativeimplementation of the zero crossing detector in each respective path(labeled 907-A and 907-B) is described below with respect to FIG. 15.

The first derivative threshold circuitry in each respective path(labeled 905-A and 905-B) operates as a positive and negative peakdetector to provide output signals that indicate the approximate timeperiods when the positive and negative peaks of the first derivativesignal provided thereto exceed predetermined thresholds (i.e., apositive peak level PPL and a negative peak level NPL). An illustrativeimplementation of such first derivative threshold circuitry 905 for thetwo different paths is described below with respect to FIGS. 14A through14C.

In the absence of noise, the occurrence of each second derivativezero-crossing indicates that the “first derivative signal” is undergoinga (positive or negative) peak which corresponds to the point in the scandata signal where a signal level transition (e.g., indicative of atransition between a space and a bar in a bar code symbol) has occurred.However, in the real-world, noise is notorious for producing falsezero-crossing detections within the second derivative zero-crossingdetection circuit. To reduce the number of “falsely detected”zero-crossings produced by noise, data gating circuit 908 is provided,which functions to gate to the binary-type A/D signal conversioncircuitry 909, only detected second derivative zero-crossings whichoccur substantially concurrent to a positive or negative peak detectedin the “first derivative signal” (as identified by the outputs signalsof the first derivative threshold circuitry 905). An example of the datagate circuitry and binary-type A/D signal conversion circuitry isdescribed below with respect to FIG. 16.

The output of the binary-type A/D conversion circuitry 909 is a digitalscan data signal D₂ having first and second (i.e. binary) signal levelswhich correspond to the bars and spaces of the bar code symbol beingscanned. Thus, the digital scan data signal D₂ appears as a pulse-widthmodulated type signal as the first and second signal levels vary inproportion to the width of bars and spaces in the scanned bar codesymbol.

The digital scan data signal D₂ is supplied to digitizing circuitry,which converts the digital scan data signal D₂, associated with eachscanned bar code symbol, into a corresponding sequence of digital words(i.e. a sequence of digital count values) D₃. Notably, in the digitalword sequence D₃, each digital word represents the time lengthassociated with each first or second signal level in the correspondingdigital scan data signal D₂. Preferably, these digital count values arein a suitable digital format for use in carrying out various symboldecoding operations which, like the scanning pattern and volume of thepresent invention, will be determined primarily by the particularscanning application at hand. Reference is made to U.S. Pat. No.5,343,027 to Knowles, incorporated herein by reference, as it providestechnical details regarding the design and construction ofmicroelectronic digitizing circuits suitable for use in the laserscanner of the present invention.

Bar code symbol decoding circuitry (which is typically implemented witha programmed microprocessor/microcontroller) receive each digital wordsequence D₃ produced from the digitizing circuit, and subject it to oneor more bar code symbol decoding algorithms in order to determine whichbar code symbol is indicated (i.e. represented) by the digital wordsequence D₃, originally derived from corresponding scan data signal D₁detected by the photodetector associated therewith.

The operation of the multi-path scan data signal processor 901 isillustrated by the signal diagrams of FIGS. 10A through 10I. FIGS. 10Aand 10B depict the signal produced at the output of the photodetector902 as the laser scanning beam scans across a bar code symbol. FIG. 10Cdepicts the output signal produced by the signal conditioning circuitry903. And FIGS. 10D through 10I depict the processing performed in one ofthe respective paths of the multi-path scan data signal processor 901.Similar processing operations with different operations characteristicsare performed in other paths of the multi-path scan data signalprocessor 901.

More specifically, each signal processing path has different operationalcharacteristics (such as different cutoff frequencies in the filteringstages of the first and second derivative signal generation circuits ofthe respective paths, different gain characteristics in amplifier stagesof the first and second derivative signal generation circuits of therespective paths, and/or different positive and negative signalthresholds in the first derivative threshold circuitry of the respectivepaths). The varying operational characteristics of the paths areoptimized to provide different signal processing functions.

For example, the cut-off frequencies in the filtering stages of thefirst and second derivative signal generation circuits of the respectivepaths can vary such that different paths minimize the paper noiseoriginating from different focal zones of the system. Alternatively,such cut-off frequencies can vary such that one or more paths maximizethe scan resolution of the system (i.e., a path with higher cutofffrequencies may be able to detect high resolution bar code symbols)while other paths minimize paper noise (i.e., a path with lower cutofffrequencies will reject paper noise from a larger frequency band abovethe selected cutoff frequencies).

In another example, the gain characteristics in the amplifier stages ofthe first and second derivative signal generation circuits of the pathsand/or the positive and negative signal thresholds in the firstderivative threshold circuitry of the paths can vary such that one ormore paths maximize the scan resolution of the system (i.e., a path withhigher gain and/or smaller positive and negative signal thresholds maybe able to detect low bar code symbols) while other paths minimize papernoise (i.e., a path with lower gain and/or larger positive and negativesignal thresholds will reject paper noise that falls below suchthresholds).

The different signal processing functions of each path of the multi-pathscan data processor as described above are preferably performed inparallel. Alternatively, the processing along each path may be performedsequentially. In this case, a programmable microcomputer may beprogrammed to dynamically activate the processing of a given path basedupon the operation of the scanner (for example, based upon the focaldistance of the scanning plane from which the scan data signal isderived, which is described in detail in U.S. application Ser. No.09/241,930, or based upon results of previous scan processing of thesystem).

By virtue of this improved architecture, the multi-path scan data signalprocessor is able to identify signal level transitions (corresponding totransitions between a space and a bar in a bar code symbol) in the scandata signal over a diverse range of operating conditions (e.g.,operating conditions where paper noise is present in addition tooperating conditions requiring high resolution scanning, such as thereading of low contrast or high resolution bar code symbols), whichenables more reliable bar code reading over such diverse operatingconditions.

Signal Conditioning Circuitry

FIGS. 11A and 11B illustrate an exemplary embodiment of the signalconditioning circuitry 903 of FIG. 9, which operates to amplify andsmooth out or otherwise filter the scan data signal produced by thephotodetector 902 to remove unwanted noise components therein. Thecircuitry 903 comprises, a number of subcomponents arranged in a serialmanner, namely: a high gain amplifier stage 1103, a multistage amplifierstage 1105, a differential amplifier stage 1107 and a low pass filter(LPF) stage 1109. The amplifier stages 1103, 1105 and 1109 amplify thevoltage of the analog scan data signal produced by the photodetector 902with gains of 90, 3.0 and 7.1, respectively, to provide a total gain ofabout 1900. The low pass filter 1109 stage operates to filter outunwanted noise in the amplified signal produced by the amplifier stages1103, 1105 and 1109. The 3 dB cutoff frequency of the low pass filtershown (which is a maximally flat Butterworth type filter) isapproximately 780 kHz, which is designed to filter out unwanted highfrequency noise (e.g., noise which lies above the expected maximumsignal frequency of 540 kHz).

The First Derivative Signal Generation Circuitry

FIG. 12 illustrates an exemplary implementation of the first derivativesignal generation circuitry 904, which is suitable for use in the twodifferent paths of the scan data signal processor of FIG. 9. As shown inFIG. 12, the first derivative signal generation circuitry 904 includes anumber of subcomponents arranged in a serial manner that process theanalog scan data signal produced by the signal conditioning circuitry903, namely: a differentiator stage 1201, a low-pass filter (LPF) stage1203, and an amplifier stage 1205.

The differentiator stage 1201 generates an signal whose voltage level isproportional to the first derivative of the analog scan data signal forthose frequencies less than the cutoff frequency of the differentiatorstage 1201, which is set by the values of R43 and C32, respectively, andcan be approximated by the expression:

${f_{c} = \frac{1}{2*\pi*R\; 43*C\; 32}},$which is approximately 3.226 MHz for the circuit elements shown.

The low pass filter stage 1203 operates to filter out unwanted noise inthe output signal produced by the differentiator stage 1201. The 3 dBcutoff frequency of the low pass filter shown (which is a maximally flatButterworth type filter) is set by the values of L5 and C36,respectively, and can be approximated by the expression:

${f_{c} = \frac{1}{2*\pi*R\; 5*C\; 36}},$which is approximately 650 kHz for the circuit elements shown.

The amplifier stage 1205 operates to amplify the voltage levels of theoutput signal produced by the LPF stage for frequencies in apredetermined frequency band. More specifically, for frequencies betweenf₁ and f₂, the amplifier produces a gain that is approximatelyproportional to R60/R54 (which is approximately 6.5 for the circuitelements shown) where:

${f_{1} = \frac{1}{2*\pi*R\; 54*C\; 39}},$which is approximately 3 kHz for the circuit elements shown.

${f_{2} = \frac{1}{2*\pi*R\; 60*C\; 43}},$which is approximately 2 MHz for the circuit elements shown. Outside thepredetermined frequency band between f₁ and f₂, the amplifier stage 1205attenuates such frequency components.

It should be noted that although the first derivative signal generationcircuitry of the two paths (labeled 904-A and 904-B in FIG. 9) share acommon function—to generate a signal approximating the first derivativeof the analog scan data signal—they may have different operationalcharacteristics that are optimized for bar code scanning and reading indiverse operating conditions.

For example, the cut-off frequencies in the differentiator stage 1201,the LPF stage 1203 and the amplifier stage 1205 of the first derivativesignal generation circuits of the respective paths (labeled 904-A and904-B) can vary (by selecting different values for the appropriatecircuit elements as set forth above) such that different paths minimizethe paper noise originating from different focal zones of the system.Techniques for selecting the appropriate cutoff frequencies thatcorrespond to the different focal zones of the laser scanning system aredescribed in detail in U.S. patent application Ser. No. 09/241,930,commonly assigned to the assignee of the present application,incorporated by reference above in its entirety. Alternatively, suchcut-off frequencies can vary such that one or more paths maximize thescan resolution of the system (i.e., a path with higher cutofffrequencies may be able to detect high resolution bar code symbols)while other paths minimize paper noise (i.e., a path with lower cutofffrequencies will reject paper noise from a larger frequency band abovethe selected cutoff frequencies).

In another example, the gain characteristics in the amplifier stage 1205of the first derivative signal generation circuits of the respectivepaths (labeled 904-A and 904B) can vary such that one path maximizes thescan resolution of the system (i.e., a path with higher gain may be ableto detect low bar code symbols) while the other path minimize papernoise (i.e., a path with lower gain will reject paper noise that mighttrigger scan errors when amplified by the high gain path).

The Second Derivative Signal Generation Circuitry

FIG. 13 illustrate an exemplary implementation of the second derivativesignal generation circuitry 906, which is suitable for use in the twodifferent paths of the scan data signal processor of FIG. 9. As shown inFIG. 13, the second derivative signal generation circuitry 906 includesa differentiator stage 1301 that generates a signal whose voltage levelis proportional to the derivative of the first derivative signalproduced by the first derivative generation circuitry 904 (thusproportional to the second derivative of the analog scan data signalproduced by the signal conditioning circuitry 903) for frequencies in apredetermined frequency band. More specifically, the differentiatorstage 1301 operates substantially as a differentiator (producing asignal whose voltage level is proportional to the derivative of thefirst derivative signal produced by the first derivative generationcircuitry 904) for frequencies less than f₁ where:

${f_{1} = \frac{1}{2*\pi*R\; 62*C\; 48}},$which is approximately 884 kHz for the circuit elements shown. Moreover,the feedback elements of the differentiator stage 1301 operatesubstantially as a low pass filter with a 3 dB cutoff frequency which isset by the values of R65 and C49, respectively, and can be approximatedby the expression:

${f_{c} = \frac{1}{2*\pi*R\; 65*C\; 49}},$which is approximately 2.15 Mhz for the circuit elements shown.For frequencies above this predetermined 3 dB cutoff frequency f_(c),the differentiator stage 1301 attenuates such frequency components.

It should be noted that although the second derivative signal generationcircuitry of the two paths (labeled 906-A and 906-B in FIG. 9) share acommon function—to generate a signal approximating the second derivativeof the analog scan data signal—they may have different operationalcharacteristics that are optimized for bar code scanning and reading indiverse operating conditions.

For example, the cut-off frequencies in the differentiator stage 1301 ofthe second derivative signal generation circuits of the respective paths(labeled 906-A and 906-B) can vary (by selecting different values forthe appropriate circuit elements as set forth above) such that differentpaths minimize the paper noise originating from different focal zones ofthe system. Techniques for selecting the appropriate cutoff frequenciesthat correspond to the different focal zones of the laser scanningsystem are described in detail in U.S. patent application Ser. No.09/241,930, commonly assigned to the assignee of the presentapplication, incorporated by reference above in its entirety.Alternatively, such cut-off frequencies can vary such that one or morepaths maximize the scan resolution of the system (i.e., a path withhigher cutoff frequencies may be able to detect high resolution bar codesymbols) while other paths minimize paper noise (i.e., a path with lowercutoff frequencies will reject paper noise from a larger frequency bandabove the selected cutoff frequencies).Zero Crossing Detector

FIG. 15 illustrates an exemplary implementation of a zero crossingdetector 907, which is suitable for use in the two different paths ofthe scan data signal processor of FIG. 9. As shown in FIG. 15, thezero-crossing detector 907 includes a comparator circuit that comparesthe second derivative signal produced from the second derivativegeneration circuit in its respective path with a zero voltage reference(i.e. the AC ground level) provided by the zero reference signalgenerator, in order to detect the occurrence of each zero-crossing inthe second derivative signal, and provide output signals (ZC_1 and ZC_2signals) identifying zero crossings in the second derivative signal.

First Derivative Signal Threshold Level Generation Circuit

FIGS. 14A through 14C illustrate exemplary implementation of the firstderivative signal threshold circuitry 905, which is suitable for use inthe two different paths of the scan data signal processor of FIG. 9. Asshown in FIGS. 14A through 14C, the first derivative signal thresholdcircuitry 905 includes an amplifier stage 1401 that amplifies thevoltage levels of the first derivative signal produced by the firstderivative signal generation circuitry 904, positive and negative peakdetectors 1403 and 1405, and a comparator stage 1407 that generatesoutput signals (e.g., the Upper_Threshold Signal and Lower_ThresholdSignal) that indicate the time period when the positive and negativepeaks of the amplified first derivative signal produced by the amplifierstage exceed predetermined thresholds (i.e., a positive peak level PPLand a negative peak level NPL). Preferably, the positive peak level PPLand negative peak level NPL are dynamic thresholds (e.g., these levelschange as the amplified analog signal changes over time) based upon a DCbias level and a percentage (portion) of the amplified first derivativesignal produced by the amplifier stage 1401. In the illustrativeembodiment shown in FIGS. 14A through 14C, capacitors C16 and C18 areconfigured as peak detectors (with a decay time constant proportional tothe values of R14/C16 and R19/C18, respectively); and the positive peaklevel PPL is set by the resistance values of the resistor networkR16,R17,R18 and R_(U) _(—) _(BIAS), while the negative peak level NPL isset by the values of the resistor network R21,R22,R23 and R_(L) _(—)_(BIAS).

It should be noted that although the first derivative signal thresholdcircuitry of the two paths (labeled 905-A and 905-B in FIG. 9) share acommon function—to generate output signals that indicate the time periodwhen the positive and negative peaks of the amplified first derivativesignal exceed predetermined thresholds—they may have differentoperational characteristics that are optimized for bar code scanning andreading in diverse operating conditions.

For example, the positive and negative peak levels in the positive andnegative peak detectors 1403 and 1405, respectively, (which are set bythe resistance values of the resistor networks therein) can vary suchthat one path maximizes the scan resolution of the system (i.e., a pathwith lower positive peak and negative peak level may be able to detectlow bar code symbols) while the other path minimize paper noise (i.e., apath with a higher positive peak and negative peak level will rejectpaper noise that that falls below such thresholds.

For example, the positive and negative peak detectors 1403 and 1405 inthe first derivative signal threshold circuitry 905-A of the first pathA may utilize a 91 kilo-ohm resistor for R_(U) _(—) _(BIAS) and R_(L)_(—) _(BIAS) of FIGS. 14A through 14C. Such resistor values produce adynamic PPL threshold which approximates 2.079 mV DC bias level plus 24%of the amplified first derivative signal, and produce a dynamic NPLthreshold which approximates a 1.921 mV DC bias level less 24% of theamplified first derivative signal. In another example, the positive andnegative peak detectors 1403 and 1405 in the first derivative signalthreshold circuitry 905-B of the second path B may utilize a 20 kilo-ohmresistor for R_(U) _(—) _(BIAS) and R_(L) _(—) _(BIAS) of FIGS. 14Athrough 14C. Such resistor values produce a dynamic PPL threshold whichapproximates a 2.316 mV DC bias level plus 21% of the amplified firstderivative signal, and produce a dynamic NPL threshold whichapproximates 1.684 mV DC bias level less 21% of the amplified firstderivative signal. Note that path A has “lower” positive peak andnegative peak levels—it may be able to detect high resolution bar codesymbols than path B. While path B has “higher” positive peak andnegative peak levels—it will reject paper noise that might trigger scanerrors in the path A).

Data Gating Circuitry and 1-Bit A/D Conversion Circuitry

FIG. 16 illustrates an exemplary implementation of the data gatingcircuitry 908 and 1-Bit A/D conversion circuitry 909, which is suitablefor use in the two different paths of the scan data signal processor ofFIG. 9. In each respective path, the data gating circuit 908 functionsto gate to the binary-type A/D signal conversion circuitry 909, onlydetected second derivative zero-crossings (identified by the outputssignals ZC_1 and ZC_2 of the zero crossing detector 907 in therespective path) which occur substantially concurrent to a positive ornegative peaks detected in the “first derivative signal” (as identifiedby the output signals—Upper_Threshold and Lower_Threshold—of the firstderivative threshold circuitry 905). As shown in FIG. 16, the data gatecircuit 908 and the 1 bit D/A conversion circuitry 909 in each path isrealized by four NAND gates (labeled 601 through 1604) configured as aset/reset latch circuit. The operation of the data gating circuitry and1 bit D/A conversion circuitry of FIG. 16 is illustrated in the signalplot of FIG. 101.

Having described illustrative embodiments of the present invention, itis understood that there a number of alternative ways to practice thepresent invention. Several different modes for carrying out the presentinvention will be described below.

For example, rather than using “analog-type” circuit technology forrealizing the signal processing subcomponents of the multi-path scandata signal processor (e.g., the differentiators, low-pass filter,amplifiers, peak detectors, data gate, etc.), it is understood that thescan data signal processing method and apparatus of the presentinvention can be implemented using digital signal processing techniquescarried out either within a programmed microcomputer or using one ormore custom or commercially available digital signal processing (DSP)chips known in the digital signal processing art.

As illustrated in FIG. 17A, when carrying out a digital implementationof the scan data signal processor of the present invention, the analogscan data signal D₁ is provided to signal conditioning circuitry 1703(which amplifies and filters the signal to remove unwanted noisecomponents as described above), whose output is provided toanalog-to-digital conversion circuitry 1705. The analog-to-digitalconversion circuitry 1705 samples the conditioned analog scan datasignals at a sampling frequency at least two times the highest frequencycomponent expected in the analog scan data signal, in accordance withthe well known Nyquist criteria, and quantizes each time-sampled scandata signal value into a discrete signal level using a suitable lengthnumber representation (e.g. 8 bits) to produce a discrete scan datasignal. A suitable quantization level can be selected in view ofexpected noise levels in the signal. Thereafter, the discrete scan datasignal is processed by the programmed processor (e.g., a digital signalprocessor 1707 and associated memory 1709 as shown) to generate asequence of digital words (i.e. a sequence of digital count values) D₃,each representing the time length associated with the signal leveltransitions in the corresponding digital scan data signal as describedabove. Preferably, these digital count values are in a suitable digitalformat for use in carrying out various symbol decoding operations which,like the scanning pattern and volume of the present invention, will bedetermined primarily by the particular scanning application at hand.

FIGS. 17B through 17D illustrate exemplary digital implementations ofthe multi-path scan data processing according to the present invention.The digital signal processing operations therein are preferably carriedout on the discrete scan data signal levels generated by the A/Dconverter 1705 and stored in the memory 1709 of FIG. 17A.

FIG. 17B illustrates exemplary digital signal processing operations thatidentify a data frame (e.g., a portion of the discrete scan data signallevels stored in memory 1709) that potentially represents a bar codesymbol (block 1723) and stores the data frame in a working buffer (block1725). Signal processing techniques that identify a data frame (withinthe discrete signal levels stored in the memory 1709) that potentiallyrepresents a bar code symbol (block 1723) are well know in the art.

FIG. 17C illustrates exemplary digital signal processing operations thatcarry out multi-path scan data signal processing according to thepresent invention. More specifically, in block 1727, a data frame isread from the working buffer. Preferably, the data frame read from theworking buffer in block 1727 was stored therein in block 1725 of FIG.17B. Alternatively, the data frame may be a block of the discrete scandata signals levels generated by the A/D converter 1705 and stored inmemory 1709 of FIG. 17A (or discrete scan data signals derivedtherefrom). The data values of the data frame are then processed by asequence of signal processing blocks (blocks 1729, 1731-1745 and1751-1765).

In block 1729, such data values are optionally interpolated (orsub-sampled). Interpolation increases the effective sampling rate of thesystem by adding data values that are derived from existing data values.Interpolation is a technique well known in the digital signal processingarts, and is discussed in great detail in Russ, “Image ProcessingHandbook,” Third Edition, IEEE Press, 1999, pg. 219-220, hereinincorporated by reference in its entirety. Sub-sampling (or decimation)decreases the effective sampling rate of the system. Sub-sampling istypically accomplished by averaging data values. Sub-sampling is atechnique well known in the digital signal processing arts, and isdiscussed in great detail in Russ, “Image Processing Handbook,” ThirdEdition, IEEE Press, 1999, pg. 166-174, herein incorporated by referencein its entirety. The resulting block of data values are provided to atleast two processing paths (for example, two paths A and B as shown).The different digital signal processing functions of each path arepreferably performed in parallel (for example, by separated threads in amulti-threaded processing system or by separate processors in amulti-processor system). Alternatively, the processing along each pathmay be performed sequentially.

In each respective processing path, the block of data values are subjectto a digital low pass filter (blocks 1731 and blocks 1753) that filterout unwanted noise. Such digital low-pass filters preferably implementone of a Butterworth-type, Chebsychev-type, MFTD-type, orelliptical-type low pass filtering transfer function, which are wellknown in the filtering art. Details of the design of such digitalfilters is set forth in the book entitled “Electrical Filter DesignHandbook,” Third Edition, by A. Williams et al. McGraw-Hill, 1996,incorporated by reference above in its entirety. The output of thedigital low pass filter (blocks 1731, 1751) is supplied to a firstderivative processing function (blocks 1733, and 1753) whichdifferentiate the filtered digital scan data signals supplied thereto.The output of the first derivative processing function (blocks 1733,1753) is normalized (blocks 1734, 1754) and supplied to a firstderivative thresholding function (blocks 1739 and 1759) and a secondderivative processing function (blocks 1735 and 1755).

The second derivative processing function (blocks 1735, 1755)differentiates the data supplied thereto to generate data representingthe second derivative of the data values read from the working buffer.Such data is supplied to a zero crossing detector function (blocks 1737,1757), which produces output data (“zero crossing data”) identifyingzero crossings in the second derivative data generated by the secondderivative function (blocks 1735, 1755).

The first derivative thresholding function (blocks 1739, 1759) operatesas a positive and negative peak detector to provide output data thatidentifies time periods when the positive and negative peaks of the datasupplied thereto exceed predetermined thresholds (i.e., a positive peaklevel PPL and a negative peak level NPL). Preferably, the positive peaklevel PPL and negative peak level NPL are dynamic thresholds (e.g.,these levels change as the digital scan data values read from theworking buffer change over time) based upon a predetermined digitalvalue and a percentage (portion) of the corresponding normalized firstderivative signal supplied thereto.

The data output of the zero crossing detector function (blocks 1737,1757) and the first derivative thresholding function (blocks 1739, 1759)are supplied to a data gate function (blocks 1741, 1761), whichfunctions to output only zero crossing data which corresponds todetected zero-crossings which occur substantially concurrent with thepositive or negative peaks detected in the normalized first derivativedata (as identified by the output data of the first derivative thresholdfunction). Thereafter, the data output by the data gate function (whichrepresents a discrete binary-level scan data signal) is supplied to abar length function (blocks 1743, 1763), which produce a digital “time”count value for each of the first and second signal levels in thediscrete binary scan data signal. Such digital count values form asequence of digital word D₃, each representing the time lengthassociated with the signal level transitions in the correspondingdigital scan data signal as described above. These digital words arestored in an output buffer (blocks 1745, 1765), for supply to aprogrammed decoder for decoding the scan data signal and producingsymbol character data string representative of the correspondinglaser-scanned bar code symbol. Alternatively, the generated discretebinary-level scan data signal can be converted back into acontinuous-type binary-level scan data signal so that it may be“digitized” using a digital signal processor of the type taught in U.S.Pat. No. 5,828,049, incorporated herein by reference.

Each digital signal processing path has different operationalcharacteristics (such as different cutoff frequencies in the low passfilters (blocks 1731 and 1751) and/or different positive and negativesignal thresholds in the first derivative threshold function (blocks1739, 1759) of the respective paths). The varying operationalcharacteristics of the paths are optimized to provide different digitalsignal processing functions.

For example, the cut-off frequencies in the low pass filters (blocks1731 and 1751) of the respective paths can vary such that differentpaths minimize the paper noise originating from different focal zones ofthe system. Alternatively, such cut-off frequencies can such that varysuch that one or more paths maximize the scan resolution of the system(i.e., a path with higher cutoff frequencies may be able to detect highresolution bar code symbols) while other paths minimize paper noise(i.e., a path with lower cutoff frequencies will reject paper noise froma larger frequency band above the selected cutoff frequencies).

In another example, the positive and negative signal thresholds in thefirst derivative threshold functions (blocks 1739, 1759) of therespective paths can vary such that one or more paths maximize the scanresolution of the system (i.e., a path with “smaller” positive andnegative signal thresholds may be able to detect low contrast bar codesymbols) while other paths minimize paper noise (i.e., a path with a“larger” positive and negative signal thresholds will reject paper noisethat falls below such thresholds.

The different digital signal processing functions of each path asdescribed above are preferably performed in parallel (for example, byseparated threads in a multi-threaded processing system or by separateprocessors in a multi-processor system). Alternatively, the processingalong each path may be performed sequentially. In this case, theprogrammable microcomputer (e.g., digital signal processing system) maybe programmed to dynamically activate the processing of a given pathbased upon the operation of the scanner (for example, based upon thefocal distance of the scanning plane from which the scan data signal isderived, which is described in detail in U.S. application Ser. No.09/241,930, or based upon results of previous scan processing of thesystem.

FIG. 17D illustrates alternative digital signal processing operationsthat carry out multi-path scan data signal processing according to thepresent invention. More specifically, in block 1771, a data frame isread from the working buffer. Preferably, the data frame read from theworking buffer in block 1727 was stored therein in block 1725 of FIG.17B. Alternatively, the data frame may be a block of the discrete scandata signals levels generated by the A/D converter 1705 and stored inmemory 1709 of FIG. 17A (or discrete scan data signals derivedtherefrom). In block 1773, such data values are optionally interpolated(or sub-sampled). Interpolation increases the effective sampling rate ofthe system by adding data values that are derived from existing datavalues.

In block 1775, the resulting block of data values are subject to adigital low pass filter that filters out unwanted noise. Such digitallow-pass filter preferably implements one of a Butterworth-type,Chebsychev-type, MFTD-type, or elliptical-type low pass filteringtransfer function, which are well known in the filtering art. Details ofthe design of such digital filters is set forth in the book entitled“Electrical Filter Design Handbook,” Third Edition, by A. Williams etal. McGraw-Hill, 1996, incorporated by reference above in its entirety.The output of the digital low pass filter (block 1775) is supplied to afirst derivative processing function (block 1777) which differentiatesthe filtered digital scan data signals supplied thereto. The output ofthe first derivative processing function (block 1777) is normalized(block 1779) and supplied to a second derivative processing function(block 1781).

The second derivative processing function (block 1781) differentiatesthe data supplied thereto to generate data representing the secondderivative of the data values read from the working buffer. Such data issupplied to a zero crossing detector function (block 1783), whichproduces output data (“zero crossing data”) identifying zero crossingsin the second derivative data generated by the second derivativefunction.

The normalized output of the first derivative processing function (block1779) is also supplied to at least one processing sub-path (for example,sub-path A as shown). In the illustrative embodiment shown in FIG. 17D,the execution of the signal processing of the second sub-path B iscontingent upon a status condition of the working buffer (e.g., whetherit has (or has not) received another full data frame. Alternatively, thedifferent digital signal processing functions of each sub-path may beperformed in parallel (for example, by separated threads in amulti-threaded processing system or by separate processors in amulti-processor system).

Each processing sub-path includes a first derivative thresholdingfunction (blocks 1785, 1795), which operates as a positive and negativepeak detector to provide output data that identifies time periods whenthe positive and negative peaks of the data supplied thereto exceedpredetermined thresholds (i.e., a positive peak level PPL and a negativepeak level NPL). Preferably, the positive peak level PPL and negativepeak level NPL are dynamic thresholds (e.g., these levels change as thedigital scan data values read from the working buffer change over time)based upon a predetermined digital value and a percentage (portion) ofthe corresponding normalized first derivative signal supplied thereto.

The data output of the zero crossing detector function (block 1783) andthe first derivative thresholding function of the respective path (block1785, 1795) are supplied to a data gate function (blocks 1787, 1797),which functions to output only zero crossing data which corresponds todetected zero-crossings which occur substantially concurrent with thepositive or negative peaks detected in the normalized first derivativedata (as identified by the output data of the first derivative thresholdfunction). Thereafter, the data output by the data gate function (whichrepresents a discrete binary-level scan data signal) is supplied to abar length function (blocks 1789, 1798), which produce a digital “time”count value for each of the first and second signal levels in thediscrete binary scan data signal. Such digital count values form asequence of digital word D₃, each representing the time lengthassociated with the signal level transitions in the correspondingdigital scan data signal as described above. These digital words arestored in an output buffer (blocks 1791, 1799), for supply to aprogrammed decoder for decoding the scan data signal and producingsymbol character data string representative of the correspondinglaser-scanned bar code symbol. Alternatively, the generated discretebinary-level scan data signal can be converted back into acontinuous-type binary-level scan data signal so that it may be“digitized” using a digital signal processor of the type taught in U.S.Pat. No. 5,828,049, incorporated herein by reference.

Each digital signal processing sub-path of FIG. 17D has differentoperational characteristics (such as different positive and negativesignal thresholds in the first derivative threshold function (blocks1785, 1795) of the respective sub-paths). The varying operationalcharacteristics of the sub-paths are optimized to provide differentdigital signal processing functions.

For example, the positive and negative signal thresholds in the firstderivative threshold functions (blocks 1785, 1795) of the respectivesub-paths can vary such that one or more sub-paths maximize the scanresolution of the system (i.e., a sub-path with “smaller” positive andnegative signal thresholds may be able to detect low bar code symbols)while other sub-paths minimize paper noise (i.e., a sub-path with a“larger” positive and negative signal thresholds will reject paper noisethat falls below such thresholds.

Note that the illustrative embodiments set forth above provide amulti-path scan data signal processor with two signal processing paths(or sub-paths) with different operational characteristics. It iscontemplated that the multi-path scan data signal processor of thepresent invention includes more than two signal processing paths (orsub-paths) with different operational characteristics as describedabove.

Advantageously, the scan data signal processor of the present inventionhas an improved signal-to-noise ratio (SNR) and dynamic range, whicheffectively increases the length of each focal zone in the system. Thisallows the system designer to provide more overlap between adjacentfocal zones or produce a laser scanning system with a larger overalldepth of field. In addition, it produces a laser scanning system capableof scanning/resolving bar code symbols having narrower element widthsand/or printed on substrates whose normal vector is disposed at largeangles from the projection axis of scanning system.

Several modifications to the illustrative embodiments have beendescribed above. It is understood, however, that various othermodifications to the illustrative embodiment of the present inventionwill readily occur to persons with ordinary skill in the art. All suchmodifications and variations are deemed to be within the scope andspirit of the present invention as defined by the accompanying Claims toInvention.

1. A point-of-sale (POS) based laser scanning system providing six-sided360-degree omni-directional bar code symbol scanning coverage at a POSstation, said POS-based laser scanning system comprising: a housingmounted in or supported on a countertop at said POS station; ahorizontal-scanning window formed in said housing, and avertical-scanning window formed in said housing; a laser scanning planegeneration mechanism disposed within a housing, for generating first andsecond pluralities of laser scanning planes which (i) intersect withinpredetermined scan regions contained within a 3-D scanning volumedefined outside of said housing, and (ii) generate a plurality of groupsof intersecting laser scanning planes within said 3-D scanning volume;wherein said laser scanning plane generation mechanism includes a firstlaser beam production module for producing a first laser beam, and afirst polygonal scanning element having multiple reflective surfaces androtating about a first axis of rotation, for scanning said first laserbeam, so as to produce a first laser scanning beam that reflects off afirst plurality of laser beam folding mirrors to generate and projectsaid first plurality of laser scanning planes through saidhorizontal-scanning window; wherein said laser scanning plane generationmechanism includes a second laser beam production module for producing asecond laser beam, and a second polygonal scanning element havingmultiple reflective surfaces and rotating about a second axis ofrotation, for scanning said second laser beam, so as to produce a secondlaser scanning beam that reflects off a second plurality of laser beamfolding mirrors to generate and project said second plurality of laserscanning planes through said vertical-scanning window; wherein saidplurality of groups of intersecting laser scanning planes form a complexomni-directional 3-D laser scanning pattern within said 3-D scanningvolume capable of scanning a bar code symbol located on the surface ofany object including a six-sided rectangular box-shaped object,presented within said 3-D scanning volume at any orientation and fromany direction at said POS station so as to provide six-sided 360-degreeomni-directional bar code symbol scanning coverage at said POS station;and wherein at least one of said plurality of groups of intersectinglaser scanning planes are projected onto each of the six sides of asix-sided rectangular box-shaped object as said six-sided rectangularbox-shaped object is passed through said 3-D scanning volume.
 2. ThePOS-based laser scanning system of claim 1, wherein said six sidescomprise the bottom-facing, top-facing, back-facing, front-facing,left-facing and right-facing surfaces of said six-sided rectangularbox-shaped object oriented within said 3-D scanning volume.
 3. ThePOS-based laser scanning system of claim 1, wherein said plurality ofgroups of intersecting laser scanning planes comprises over sixty (60)different laser scanning planes cooperating within said 3-D scanningvolume to generate said complex omni-directional 3-D laser scanningpattern.
 4. The POS-based laser scanning system of claim 1, wherein eachsaid group of intersecting laser scanning planes comprises: (i) aplurality of substantially-vertical laser scanning planes for readingbar code symbols having bar code elements that are orientedsubstantially parallel with respect to said horizontal-scanning window,and (ii) a plurality of substantially-horizontal laser scanning planesfor reading bar code symbols having bar code elements that are orientedsubstantially orthogonal with respect to said horizontal-scanningwindow.
 5. The POS-based laser scanning system of claim 1, wherein saidfirst laser beam production module comprises a first visible laser diode(VLD), and said second laser beam production module comprises a secondvisible laser diode (VLD).
 6. The POS-based laser scanning system ofclaim 1, wherein said first plurality of laser beam folding mirrors andsaid first laser beam production module cooperate with first and secondlight collecting/focusing optical elements and first and secondphotodetectors disposed within said housing to form first and secondscanning stations disposed about said first polygonal scanning element,and wherein the light collecting/focusing optical element within eachsaid first and second laser scanning station collects light frompredetermined scan regions within said 3-D scanning volume and focusessuch collected light onto the photodetector to produce an electricalsignal having an amplitude proportional to the intensity of lightfocused thereon, and said electrical signal being supplied toanalog/digital signal processing circuitry for processing analog anddigital scan data signals derived therefrom to perform bar code symbolreading operations.
 7. The POS-based laser scanning system of claim 6,wherein said second plurality of laser beam folding mirrors and saidsecond laser beam production module cooperate with a third lightcollecting/focusing optical element and a third photodetector disposedwithin said housing to form said third scanning station disposed aboutsaid second polygonal scanning element, and wherein the lightcollecting/focusing optical element within said third laser scanningstation collects light from predetermined scan regions within said 3-Dscanning volume and focuses such collected light onto the photodetectorto produce an electrical signal having an amplitude proportional to theintensity of light focused thereon, and said electrical signal beingsupplied to analog/digital signal processing circuitry for processinganalog and digital scan data signals derived therefrom to perform barcode symbol reading operations.
 8. The POS-based laser scanning systemof claim 1, wherein said first polygonal scanning element comprises afirst polygonal scanning mirror having a first plurality of rotatingmirror facets, and wherein said second polygonal scanning elementcomprises a second polygonal scanning mirror having a second pluralityof rotating mirror facets.
 9. The POS-based laser scanning system ofclaim 8, wherein said second plurality of rotating mirror facets on saidsecond polygonal scanning mirror are classifiable into a first class offacets having High Elevation (HE) angle characteristics, and a secondclass of facets having Low Elevation (LE) angle characteristics.
 10. ThePOS-based laser scanning system of claim 1, wherein said complexomni-directional 3-D laser scanning pattern is generated during eachrevolution of said first and second polygonal scanning elements.